runtime: avoid reference to stale stack after GC shrinkstack
runtime: avoid reference to stale stack after GC shrinkstack

Dangling pointer error. Unlikely to trigger in practice, but still.
Found by running GODEBUG=efence=1 GOGC=1 trace.test.

Change-Id: Ice474dedcf62dd33ab77526287a023ba3b166db9
Reviewed-on: https://go-review.googlesource.com/12991
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

baiyunping333/go,albertjin/golang-go,ganboing/go-esx,monetate/go,Samurais/go,miolini/go,kevinc0825/go,zcwfeng/go,ifagnosticism/go,akutz/go,ErikDubbelboer/go,myPublicGit/go,malvira/go,Samurais/go,cyberroadie/go,skotti/go,cookerzhu/go,chinanjjohn2012/go,akutz/go,samanalysis/go,linux-on-ibm-z/go,vanloswang/go,zouchao2010/go,skotti/go,jameskumar/go,mikekap/go,AALEKH/go,kakuhiroshi/go,springning/go,codestation/go,xiaobodu/go,jasonxiong/go,jonasi/go,vanloswang/go,miolini/go,rachtsingh/gogojuice,binarycrusader/go,polyverse-security/go,cainiaocome/go,kim-racktop/go,willjunspecial/go,CarterTsai/go,akutz/go,chinanjjohn2012/go,mikekap/go,ganboing/go-esx,mbrukman/go,Stan-Lin-/go,roth1002/go,dafyddcrosby/go,GeorgiCodes/go,jameskumar/go,XuYunnan/go,jdhenke/go,polyverse-security/go,danielsoro/go,jameskumar/go,skotti/go,yuhengye/go,jacobhaven/go,myPublicGit/go,JioCloud/go,WIZARD-CXY/go,WIZARD-CXY/go,rachtsingh/gogojuice,yangzhongj/go,NunoEdgarGub1/go,sndnvaps/go,jmptrader/go,willjunspecial/go,stormltf/go,wangfakang/go-1,beealone/go,mwhudson/go,malvira/go,suncycheng/go,momchil-velikov/go,XuYunnan/go,Jordanzuo/go,ichu501/go,NunoEdgarGub1/go,CAFxX/go,jonathanmarvens/go,miolini/go,Badredapple/go,devpadawan/go,b54898533/go,Cofyc/go,alex-zhang/go,danycoro/go,alex-zhang/go,GeorgiCodes/go,mdp/go,zhangg/go,jdhenke/go,fjballest/golang,vanloswang/go,constantine001/go,qskycolor/go,sndnvaps/go,kim-racktop/go,xiaobodu/go,sndnvaps/go,wangfakang/go-1,parkdy/go,yuhengye/go,polyverse-security/go,NunoEdgarGub1/go,golang/go,ErikDubbelboer/go,likesea/go,ganboing/go-esx,ichu501/go,xujianhai/go,jmptrader/go,honsiorovskyi/go,dterei/go,binarycrusader/go,parkdy/go,kim-racktop/go,greyhwndz/go,Badredapple/go,g522342435/go,momchil-velikov/go,zcwfeng/go,cyberroadie/go,fjballest/golang,samanalysis/go,b54898533/go,andrewchambers/go,miolini/go,nakedgc/go,cyberroadie/go,4ad/go,Cofyc/go,jasonxiong/go,alex-zhang/go,beealone/go,ericsnowcurrently/go,codestation/go,jmptrader/go,goodlang/good,zouchao2010/go,jameskumar/go,Cofyc/go,binarycrusader/go,b54898533/go,odeke-em/go,theif519/go,vsekhar/elastic-go,jonahglover/go,Badredapple/go,myPublicGit/go,devpadawan/go,xiaobodu/go,joshsammut/go,kevinc0825/go,suncycheng/go,ObjectiveJoe/go,jonasi/go,nakedgc/go,JioCloud/go,XuYunnan/go,vsekhar/elastic-go,dpatel06/go,zcwfeng/go,xiuzhifu/go,jdhenke/go,CiscoCloud/go,baiyunping333/go,qskycolor/go,alash3al/go,skatsuta/go,Charlesdong/go-1,NichoZhang/go,monetate/go,snowsnail/go,ifagnosticism/go,famorted/go,beealone/go,yasarkunduz/go,parkdy/go,josharian/go,lihuanghai/go,arschles/golang-go,danielsoro/go,jefby/go,cyberroadie/go,dafyddcrosby/go,wangfakang/go-1,NunoEdgarGub1/go,ifagnosticism/go,goodlang/good,xujianhai/go,xiaobodu/go,b54898533/go,Badredapple/go,CarterTsai/go,CodyGuo/go,gdey/go,Endika/go,rentongzhang/go,jacobhaven/go,Stan-Lin-/go,Stan-Lin-/go,vanloswang/go,CiscoCloud/go,4ad/go,jacobhaven/go,myPublicGit/go,vishsingh/go,parkdy/go,Young55555/go,mwhudson/go,stormltf/go,fjballest/golang,zhangg/go,goodlang/good,goodlang/good,Charlesdong/go-1,jonathanmarvens/go,polyverse-security/go,jacobhaven/go,ErikDubbelboer/go,CAFxX/go,CAFxX/go,Samurais/go,likesea/go,suncycheng/go,mwhudson/go,NichoZhang/go,AALEKH/go,jacobhaven/go,malvira/go,cainiaocome/go,skatsuta/go,jdeng/go,rentongzhang/go,ImJasonH/go,ObjectiveJoe/go,ganboing/go-esx,cloudflare/go,theif519/go,arschles/go-1,Pulgafree/go,kevinc0825/go,nakedgc/go,gdey/go,vsekhar/elastic-go,AALEKH/go,arschles/golang-go,codestation/go,xiaobodu/go,NichoZhang/go,Samurais/go,honsiorovskyi/go,ericsnowcurrently/go,josharian/go,sndnvaps/go,snowsnail/go,ganboing/go-esx,jonahglover/go,kakuhiroshi/go,rentongzhang/go,uileyar/go,ichu501/go,lihuanghai/go,malvira/go,beealone/go,sosop/go,polyverse-security/go,goodlang/good,honsiorovskyi/go,vsdutka/go,Samurais/go,vishsingh/go,yangzhongj/go,golang/go,wangfakang/go-1,Pulgafree/go,jacobsa/go,nakedgc/go,xiuzhifu/go,qskycolor/go,ichu501/go,NichoZhang/go,arschles/go-1,jdhenke/go,GeorgiCodes/go,dafyddcrosby/go,Jordanzuo/go,cainiaocome/go,lihuanghai/go,miolini/go,josharian/go,shines77/go,roth1002/go,vishsingh/go,jdeng/go,NichoZhang/go,qskycolor/go,sinuos/go,chinanjjohn2012/go,willjunspecial/go,arschles/go-1,danielsoro/go,slavau/go,roth1002/go,undergrowthlinear/go,uileyar/go,honsiorovskyi/go,ObjectiveJoe/go,ErikDubbelboer/go,4ad/go,Cofyc/go,shines77/go,beealone/go,momchil-velikov/go,suncycheng/go,CAFxX/go,Stan-Lin-/go,cookerzhu/go,Endika/go,leeric92/go,jacobxk/go,xiuzhifu/go,vsekhar/elastic-go,rachtsingh/gogojuice,codestation/go,jacobxk/go,ichu501/go,beealone/go,skotti/go,dafyddcrosby/go,NichoZhang/go,goodlang/good,nakedgc/go,mwhudson/go,arschles/go-1,dterei/go,cyberroadie/go,GeorgiCodes/go,codestation/go,ErikDubbelboer/go,akutz/go,albertjin/golang-go,JioCloud/go,deft-code/go,snowsnail/go,cainiaocome/go,joshsammut/go,xujianhai/go,theass/go,Charlesdong/go-1,xiuzhifu/go,baiyunping333/go,parkdy/go,willjunspecial/go,deft-code/go,jdhenke/go,baiyunping333/go,jonahglover/go,cloudflare/go,dpatel06/go,Cofyc/go,vsdutka/go,jmptrader/go,alash3al/go,josharian/go,undergrowthlinear/go,ifagnosticism/go,jonasi/go,linux-on-ibm-z/go,youprofit/go,ImJasonH/go,andrewchambers/go,parkdy/go,Jordanzuo/go,mikekap/go,dterei/go,miolini/go,roth1002/go,odeke-em/go,jacobxk/go,g522342435/go,kim-racktop/go,cainiaocome/go,jonahglover/go,sndnvaps/go,vsdutka/go,undergrowthlinear/go,golang/go,snowsnail/go,sinuos/go,jefby/go,deft-code/go,ichu501/go,springning/go,rentongzhang/go,xujianhai/go,stormltf/go,mbrukman/go,mdp/go,samanalysis/go,Stan-Lin-/go,derekmarcotte/go,jdeng/go,CarterTsai/go,myPublicGit/go,mdp/go,dterei/go,jonahglover/go,slavau/go,yangzhongj/go,XuYunnan/go,honsiorovskyi/go,WIZARD-CXY/go,jefby/go,jonathanmarvens/go,deft-code/go,yuhengye/go,cyberroadie/go,kevinc0825/go,golang/go,ganboing/go-esx,odeke-em/go,jefby/go,yangzhongj/go,monetate/go,Stan-Lin-/go,mbrukman/go,greyhwndz/go,jdhenke/go,jefby/go,vsekhar/elastic-go,NunoEdgarGub1/go,stormltf/go,polyverse-security/go,binarycrusader/go,wangfakang/go-1,undergrowthlinear/go,cloudflare/go,devpadawan/go,cyberroadie/go,mwhudson/go,rachtsingh/gogojuice,mbrukman/go,dterei/go,likesea/go,malvira/go,theass/go,greyhwndz/go,Endika/go,kakuhiroshi/go,miolini/go,donnel6809/go,deft-code/go,famorted/go,jacobsa/go,andrewchambers/go,Charlesdong/go-1,samanalysis/go,WIZARD-CXY/go,mbrukman/go,Young55555/go,shines77/go,miolini/go,kakuhiroshi/go,donnel6809/go,zhangg/go,springning/go,rentongzhang/go,albertjin/golang-go,Cofyc/go,CAFxX/go,zouchao2010/go,springning/go,mikekap/go,ImJasonH/go,gdey/go,ericsnowcurrently/go,zcwfeng/go,arschles/golang-go,codestation/go,parkdy/go,danycoro/go,jonathanmarvens/go,springning/go,wangfakang/go-1,odeke-em/go,ObjectiveJoe/go,jacobsa/go,albertjin/golang-go,ericsnowcurrently/go,vanloswang/go,cloudflare/go,GeorgiCodes/go,odeke-em/go,ifagnosticism/go,ganboing/go-esx,binarycrusader/go,ImJasonH/go,albertjin/golang-go,youprofit/go,dafyddcrosby/go,slavau/go,cainiaocome/go,zouchao2010/go,danycoro/go,sosop/go,dpatel06/go,Pulgafree/go,donnel6809/go,gdey/go,ObjectiveJoe/go,Samurais/go,suncycheng/go,rentongzhang/go,Samurais/go,mgyenik/go,youprofit/go,xiuzhifu/go,jmptrader/go,cainiaocome/go,binarycrusader/go,mdp/go,CodyGuo/go,cloudflare/go,lihuanghai/go,Young55555/go,sndnvaps/go,kevinc0825/go,theif519/go,jonathanmarvens/go,kakuhiroshi/go,devpadawan/go,CodyGuo/go,golang/go,vishsingh/go,danycoro/go,albertjin/golang-go,kakuhiroshi/go,derekmarcotte/go,Charlesdong/go-1,kim-racktop/go,jdeng/go,Charlesdong/go-1,andrewchambers/go,leeric92/go,jonasi/go,Cofyc/go,jefby/go,g522342435/go,theass/go,uileyar/go,skotti/go,willjunspecial/go,myPublicGit/go,AALEKH/go,CiscoCloud/go,jdeng/go,akutz/go,stormltf/go,youprofit/go,jdeng/go,skatsuta/go,jacobxk/go,Badredapple/go,linux-on-ibm-z/go,nawawi/go,christopher-henderson/Go,ObjectiveJoe/go,alex-zhang/go,zhangg/go,mikekap/go,ifagnosticism/go,dafyddcrosby/go,vishsingh/go,g522342435/go,danielsoro/go,deft-code/go,nawawi/go,sosop/go,jacobhaven/go,mgyenik/go,akutz/go,danielsoro/go,NunoEdgarGub1/go,jacobsa/go,gdey/go,ImJasonH/go,likesea/go,derekmarcotte/go,jasonxiong/go,zhangg/go,jasonxiong/go,momchil-velikov/go,jonahglover/go,yuhengye/go,cloudflare/go,jonasi/go,dpatel06/go,zhangg/go,sinuos/go,JioCloud/go,uileyar/go,likesea/go,jacobxk/go,donnel6809/go,greyhwndz/go,Jordanzuo/go,b54898533/go,andrewchambers/go,ichu501/go,mdp/go,snowsnail/go,devpadawan/go,mdp/go,undergrowthlinear/go,ImJasonH/go,xiuzhifu/go,arschles/golang-go,rentongzhang/go,Badredapple/go,yangzhongj/go,CodyGuo/go,constantine001/go,zhangg/go,danycoro/go,momchil-velikov/go,wangfakang/go-1,CarterTsai/go,leeric92/go,arschles/go-1,skotti/go,ObjectiveJoe/go,jasonxiong/go,JioCloud/go,springning/go,alash3al/go,ErikDubbelboer/go,malvira/go,springning/go,arschles/go-1,arschles/go-1,sinuos/go,CarterTsai/go,rachtsingh/gogojuice,sosop/go,xujianhai/go,devpadawan/go,CiscoCloud/go,famorted/go,constantine001/go,shines77/go,yasarkunduz/go,zouchao2010/go,CarterTsai/go,leeric92/go,lihuanghai/go,dterei/go,famorted/go,ImJasonH/go,Endika/go,CodyGuo/go,undergrowthlinear/go,rachtsingh/gogojuice,cloudflare/go,zcwfeng/go,GeorgiCodes/go,derekmarcotte/go,leeric92/go,leeric92/go,jdeng/go,mdp/go,jmptrader/go,alex-zhang/go,kakuhiroshi/go,mwhudson/go,jdeng/go,ericsnowcurrently/go,yasarkunduz/go,samanalysis/go,theass/go,beealone/go,JioCloud/go,josharian/go,vanloswang/go,g522342435/go,jonathanmarvens/go,chinanjjohn2012/go,b54898533/go,uileyar/go,fjballest/golang,mgyenik/go,ImJasonH/go,yuhengye/go,4ad/go,qskycolor/go,monetate/go,sndnvaps/go,ericsnowcurrently/go,likesea/go,NichoZhang/go,deft-code/go,xiuzhifu/go,nawawi/go,Pulgafree/go,GeorgiCodes/go,Badredapple/go,Jordanzuo/go,skatsuta/go,fjballest/golang,xiaobodu/go,CAFxX/go,golang/go,nawawi/go,yangzhongj/go,xiaobodu/go,XuYunnan/go,jacobsa/go,greyhwndz/go,Pulgafree/go,akutz/go,baiyunping333/go,cainiaocome/go,vishsingh/go,springning/go,linux-on-ibm-z/go,CarterTsai/go,skatsuta/go,ObjectiveJoe/go,xujianhai/go,lihuanghai/go,shines77/go,4ad/go,WIZARD-CXY/go,zcwfeng/go,binarycrusader/go,yangzhongj/go,undergrowthlinear/go,dterei/go,Pulgafree/go,jonahglover/go,Endika/go,GeorgiCodes/go,xujianhai/go,Jordanzuo/go,jonasi/go,stormltf/go,CodyGuo/go,leeric92/go,samanalysis/go,yuhengye/go,yasarkunduz/go,dafyddcrosby/go,slavau/go,youprofit/go,JioCloud/go,AALEKH/go,xiaobodu/go,Pulgafree/go,CodyGuo/go,likesea/go,uileyar/go,mwhudson/go,theass/go,joshsammut/go,jonahglover/go,CiscoCloud/go,linux-on-ibm-z/go,alex-zhang/go,cookerzhu/go,mbrukman/go,Endika/go,jacobsa/go,nawawi/go,andrewchambers/go,shines77/go,xiuzhifu/go,mikekap/go,derekmarcotte/go,WIZARD-CXY/go,roth1002/go,stormltf/go,vsdutka/go,kim-racktop/go,kakuhiroshi/go,mgyenik/go,vsekhar/elastic-go,XuYunnan/go,jonasi/go,zcwfeng/go,alash3al/go,derekmarcotte/go,4ad/go,ericsnowcurrently/go,devpadawan/go,cookerzhu/go,CiscoCloud/go,qskycolor/go,dpatel06/go,joshsammut/go,Badredapple/go,zcwfeng/go,vsdutka/go,yasarkunduz/go,malvira/go,ifagnosticism/go,fjballest/golang,Young55555/go,albertjin/golang-go,mikekap/go,kevinc0825/go,youprofit/go,jdhenke/go,nakedgc/go,4ad/go,vsdutka/go,vsekhar/elastic-go,cloudflare/go,sinuos/go,danycoro/go,g522342435/go,kim-racktop/go,mgyenik/go,sosop/go,honsiorovskyi/go,nawawi/go,skotti/go,myPublicGit/go,donnel6809/go,zouchao2010/go,alash3al/go,cyberroadie/go,mgyenik/go,yasarkunduz/go,arschles/golang-go,binarycrusader/go,shines77/go,linux-on-ibm-z/go,slavau/go,Cofyc/go,odeke-em/go,4ad/go,rentongzhang/go,golang/go,chinanjjohn2012/go,roth1002/go,famorted/go,cookerzhu/go,jasonxiong/go,sosop/go,monetate/go,zouchao2010/go,uileyar/go,donnel6809/go,greyhwndz/go,roth1002/go,AALEKH/go,andrewchambers/go,kevinc0825/go,Young55555/go,Stan-Lin-/go,youprofit/go,nakedgc/go,danielsoro/go,monetate/go,slavau/go,dafyddcrosby/go,fjballest/golang,NichoZhang/go,CAFxX/go,ichu501/go,nakedgc/go,chinanjjohn2012/go,vanloswang/go,greyhwndz/go,samanalysis/go,zhangg/go,skatsuta/go,g522342435/go,jdhenke/go,monetate/go,jameskumar/go,NunoEdgarGub1/go,codestation/go,jacobsa/go,odeke-em/go,constantine001/go,albertjin/golang-go,ifagnosticism/go,polyverse-security/go,danycoro/go,goodlang/good,deft-code/go,willjunspecial/go,arschles/golang-go,sosop/go,theif519/go,alash3al/go,dpatel06/go,monetate/go,jonathanmarvens/go,dterei/go,jacobxk/go,Stan-Lin-/go,golang/go,josharian/go,suncycheng/go,suncycheng/go,fjballest/golang,snowsnail/go,sinuos/go,devpadawan/go,AALEKH/go,Jordanzuo/go,beealone/go,honsiorovskyi/go,donnel6809/go,jasonxiong/go,chinanjjohn2012/go,arschles/golang-go,sosop/go,alex-zhang/go,jacobxk/go,alash3al/go,mgyenik/go,sndnvaps/go,jameskumar/go,Young55555/go,Young55555/go,andrewchambers/go,linux-on-ibm-z/go,momchil-velikov/go,cookerzhu/go,suncycheng/go,qskycolor/go,WIZARD-CXY/go,mikekap/go,mbrukman/go,joshsammut/go,Samurais/go,myPublicGit/go,theif519/go,samanalysis/go,arschles/golang-go,constantine001/go,jonathanmarvens/go,willjunspecial/go,jonasi/go,slavau/go,CarterTsai/go,vishsingh/go,alex-zhang/go,yuhengye/go,danielsoro/go,famorted/go,slavau/go,jmptrader/go,gdey/go,joshsammut/go,danycoro/go,sinuos/go,lihuanghai/go,uileyar/go,yangzhongj/go,willjunspecial/go,derekmarcotte/go,theif519/go,nawawi/go,CodyGuo/go,josharian/go,joshsammut/go,stormltf/go,rachtsingh/gogojuice,parkdy/go,skotti/go,jameskumar/go,mwhudson/go,yuhengye/go,NunoEdgarGub1/go,derekmarcotte/go,wangfakang/go-1,danielsoro/go,vsdutka/go,famorted/go,donnel6809/go,joshsammut/go,goodlang/good,XuYunnan/go,theass/go,snowsnail/go,jefby/go,baiyunping333/go,famorted/go,gdey/go,Charlesdong/go-1,CAFxX/go,Charlesdong/go-1,chinanjjohn2012/go,ganboing/go-esx,snowsnail/go,codestation/go,constantine001/go,undergrowthlinear/go,baiyunping333/go,jacobsa/go,ErikDubbelboer/go,honsiorovskyi/go,qskycolor/go,akutz/go,nawawi/go,youprofit/go,mdp/go,alash3al/go,jefby/go,dpatel06/go,zouchao2010/go,cookerzhu/go,polyverse-security/go,b54898533/go,constantine001/go,Jordanzuo/go,CiscoCloud/go,skatsuta/go,odeke-em/go,roth1002/go,theass/go,gdey/go,rachtsingh/gogojuice,yasarkunduz/go,b54898533/go,jacobxk/go,momchil-velikov/go,momchil-velikov/go,lihuanghai/go,vishsingh/go,vanloswang/go,baiyunping333/go,vsdutka/go,AALEKH/go,josharian/go,Young55555/go,jacobhaven/go,dpatel06/go,theif519/go,shines77/go,kevinc0825/go,jameskumar/go,vsekhar/elastic-go,likesea/go,Pulgafree/go,leeric92/go,xujianhai/go,JioCloud/go,jmptrader/go,yasarkunduz/go,mgyenik/go,theass/go,ericsnowcurrently/go,linux-on-ibm-z/go,Endika/go,ErikDubbelboer/go,greyhwndz/go,cookerzhu/go,skatsuta/go,Endika/go,sinuos/go,constantine001/go,jasonxiong/go,kim-racktop/go,jacobhaven/go,theif519/go,mbrukman/go,g522342435/go,malvira/go,arschles/go-1,CiscoCloud/go,WIZARD-CXY/go,XuYunnan/go