Protect against dereferencing a null ptr.
Protect against dereferencing a null ptr.

'stream' may be left nullptr after a pop, so don't try to dereference
it.

(via nlewycky@google.com).

kskalski/grpc,philcleveland/grpc,kskalski/grpc,infinit/grpc,soltanmm/grpc,philcleveland/grpc,leifurhauks/grpc,yinsu/grpc,vsco/grpc,fuchsia-mirror/third_party-grpc,andrewpollock/grpc,7anner/grpc,yongni/grpc,greasypizza/grpc,bogdandrutu/grpc,tamihiro/grpc,philcleveland/grpc,firebase/grpc,ejona86/grpc,grani/grpc,podsvirov/grpc,msiedlarek/grpc,miselin/grpc,soltanmm/grpc,LuminateWireless/grpc,wangyikai/grpc,firebase/grpc,hstefan/grpc,7anner/grpc,MakMukhi/grpc,a11r/grpc,jboeuf/grpc,maxwell-demon/grpc,royalharsh/grpc,murgatroid99/grpc,kskalski/grpc,quizlet/grpc,nicolasnoble/grpc,thinkerou/grpc,geffzhang/grpc,podsvirov/grpc,maxwell-demon/grpc,fuchsia-mirror/third_party-grpc,simonkuang/grpc,chrisdunelm/grpc,rjshade/grpc,pszemus/grpc,dklempner/grpc,ncteisen/grpc,kpayson64/grpc,cgvarela/grpc,simonkuang/grpc,ppietrasa/grpc,deepaklukose/grpc,soltanmm-google/grpc,pszemus/grpc,Crevil/grpc,jcanizales/grpc,miselin/grpc,a11r/grpc,ncteisen/grpc,msiedlarek/grpc,baylabs/grpc,daniel-j-born/grpc,hstefan/grpc,dgquintas/grpc,vjpai/grpc,tamihiro/grpc,jboeuf/grpc,jboeuf/grpc,jboeuf/grpc,thunderboltsid/grpc,ananthonline/grpc,msiedlarek/grpc,maxwell-demon/grpc,ncteisen/grpc,tamihiro/grpc,sreecha/grpc,donnadionne/grpc,donnadionne/grpc,sreecha/grpc,makdharma/grpc,grani/grpc,matt-kwong/grpc,nicolasnoble/grpc,soltanmm/grpc,madongfly/grpc,jcanizales/grpc,ofrobots/grpc,ejona86/grpc,ofrobots/grpc,thinkerou/grpc,perumaalgoog/grpc,kpayson64/grpc,LuminateWireless/grpc,yang-g/grpc,royalharsh/grpc,dgquintas/grpc,miselin/grpc,quizlet/grpc,philcleveland/grpc,miselin/grpc,perumaalgoog/grpc,murgatroid99/grpc,MakMukhi/grpc,7anner/grpc,firebase/grpc,rjshade/grpc,deepaklukose/grpc,larsonmpdx/grpc,a11r/grpc,mehrdada/grpc,Vizerai/grpc,goldenbull/grpc,ejona86/grpc,ncteisen/grpc,carl-mastrangelo/grpc,LuminateWireless/grpc,sreecha/grpc,hstefan/grpc,makdharma/grpc,goldenbull/grpc,chrisdunelm/grpc,zhimingxie/grpc,JoeWoo/grpc,mehrdada/grpc,wcevans/grpc,baylabs/grpc,a11r/grpc,apolcyn/grpc,greasypizza/grpc,jboeuf/grpc,a-veitch/grpc,baylabs/grpc,miselin/grpc,maxwell-demon/grpc,ananthonline/grpc,ppietrasa/grpc,miselin/grpc,podsvirov/grpc,leifurhauks/grpc,podsvirov/grpc,carl-mastrangelo/grpc,muxi/grpc,JoeWoo/grpc,LuminateWireless/grpc,cgvarela/grpc,Vizerai/grpc,baylabs/grpc,a-veitch/grpc,ipylypiv/grpc,grani/grpc,thunderboltsid/grpc,royalharsh/grpc,zhimingxie/grpc,kriswuollett/grpc,simonkuang/grpc,pmarks-net/grpc,bjori/grpc,pszemus/grpc,jcanizales/grpc,firebase/grpc,carl-mastrangelo/grpc,muxi/grpc,infinit/grpc,malexzx/grpc,thinkerou/grpc,andrewpollock/grpc,chrisdunelm/grpc,pszemus/grpc,doubi-workshop/grpc,ncteisen/grpc,dklempner/grpc,makdharma/grpc,kumaralokgithub/grpc,matt-kwong/grpc,madongfly/grpc,JoeWoo/grpc,grpc/grpc,PeterFaiman/ruby-grpc-minimal,goldenbull/grpc,matt-kwong/grpc,jcanizales/grpc,ofrobots/grpc,geffzhang/grpc,ofrobots/grpc,yang-g/grpc,perumaalgoog/grpc,yugui/grpc,muxi/grpc,jtattermusch/grpc,apolcyn/grpc,adelez/grpc,hstefan/grpc,kpayson64/grpc,pszemus/grpc,w4-sjcho/grpc,LuminateWireless/grpc,VcamX/grpc,arkmaxim/grpc,y-zeng/grpc,a-veitch/grpc,grani/grpc,baylabs/grpc,ctiller/grpc,jtattermusch/grpc,quizlet/grpc,yongni/grpc,deepaklukose/grpc,leifurhauks/grpc,thunderboltsid/grpc,mehrdada/grpc,kumaralokgithub/grpc,wangyikai/grpc,apolcyn/grpc,ppietrasa/grpc,msiedlarek/grpc,Vizerai/grpc,muxi/grpc,pszemus/grpc,ananthonline/grpc,kskalski/grpc,arkmaxim/grpc,goldenbull/grpc,pszemus/grpc,donnadionne/grpc,chrisdunelm/grpc,greasypizza/grpc,bogdandrutu/grpc,carl-mastrangelo/grpc,apolcyn/grpc,vsco/grpc,ipylypiv/grpc,a-veitch/grpc,infinit/grpc,w4-sjcho/grpc,kpayson64/grpc,sreecha/grpc,ctiller/grpc,podsvirov/grpc,geffzhang/grpc,yang-g/grpc,y-zeng/grpc,tengyifei/grpc,ipylypiv/grpc,VcamX/grpc,ejona86/grpc,kriswuollett/grpc,soltanmm/grpc,quizlet/grpc,tengyifei/grpc,malexzx/grpc,murgatroid99/grpc,andrewpollock/grpc,nicolasnoble/grpc,infinit/grpc,daniel-j-born/grpc,stanley-cheung/grpc,fuchsia-mirror/third_party-grpc,pmarks-net/grpc,dgquintas/grpc,zhimingxie/grpc,dklempner/grpc,grpc/grpc,bogdandrutu/grpc,wcevans/grpc,larsonmpdx/grpc,andrewpollock/grpc,apolcyn/grpc,apolcyn/grpc,fuchsia-mirror/third_party-grpc,soltanmm/grpc,leifurhauks/grpc,dklempner/grpc,jtattermusch/grpc,madongfly/grpc,larsonmpdx/grpc,donnadionne/grpc,murgatroid99/grpc,cgvarela/grpc,vsco/grpc,a11r/grpc,ncteisen/grpc,matt-kwong/grpc,tengyifei/grpc,kumaralokgithub/grpc,jboeuf/grpc,sreecha/grpc,ctiller/grpc,dgquintas/grpc,apolcyn/grpc,fuchsia-mirror/third_party-grpc,yongni/grpc,soltanmm-google/grpc,soltanmm/grpc,murgatroid99/grpc,thinkerou/grpc,msiedlarek/grpc,thinkerou/grpc,msmania/grpc,pmarks-net/grpc,ejona86/grpc,philcleveland/grpc,geffzhang/grpc,PeterFaiman/ruby-grpc-minimal,adelez/grpc,w4-sjcho/grpc,bjori/grpc,yugui/grpc,sreecha/grpc,jboeuf/grpc,murgatroid99/grpc,arkmaxim/grpc,chrisdunelm/grpc,jcanizales/grpc,carl-mastrangelo/grpc,doubi-workshop/grpc,simonkuang/grpc,jboeuf/grpc,tengyifei/grpc,madongfly/grpc,hstefan/grpc,cgvarela/grpc,msiedlarek/grpc,firebase/grpc,muxi/grpc,vjpai/grpc,kpayson64/grpc,wangyikai/grpc,firebase/grpc,MakMukhi/grpc,yugui/grpc,ipylypiv/grpc,LuminateWireless/grpc,yang-g/grpc,infinit/grpc,wangyikai/grpc,daniel-j-born/grpc,baylabs/grpc,kskalski/grpc,goldenbull/grpc,VcamX/grpc,nicolasnoble/grpc,thinkerou/grpc,wcevans/grpc,jtattermusch/grpc,jcanizales/grpc,geffzhang/grpc,thinkerou/grpc,ppietrasa/grpc,fuchsia-mirror/third_party-grpc,MakMukhi/grpc,kpayson64/grpc,MakMukhi/grpc,MakMukhi/grpc,donnadionne/grpc,murgatroid99/grpc,ncteisen/grpc,soltanmm-google/grpc,carl-mastrangelo/grpc,w4-sjcho/grpc,kriswuollett/grpc,grpc/grpc,PeterFaiman/ruby-grpc-minimal,perumaalgoog/grpc,donnadionne/grpc,kpayson64/grpc,rjshade/grpc,stanley-cheung/grpc,rjshade/grpc,larsonmpdx/grpc,chrisdunelm/grpc,soltanmm/grpc,kskalski/grpc,jtattermusch/grpc,vjpai/grpc,podsvirov/grpc,bogdandrutu/grpc,grani/grpc,leifurhauks/grpc,ofrobots/grpc,kriswuollett/grpc,a11r/grpc,goldenbull/grpc,goldenbull/grpc,matt-kwong/grpc,stanley-cheung/grpc,arkmaxim/grpc,podsvirov/grpc,yang-g/grpc,soltanmm-google/grpc,jcanizales/grpc,VcamX/grpc,Vizerai/grpc,fuchsia-mirror/third_party-grpc,adelez/grpc,yongni/grpc,y-zeng/grpc,vsco/grpc,kskalski/grpc,vsco/grpc,Vizerai/grpc,daniel-j-born/grpc,podsvirov/grpc,infinit/grpc,grani/grpc,pmarks-net/grpc,malexzx/grpc,soltanmm-google/grpc,dklempner/grpc,stanley-cheung/grpc,wcevans/grpc,Vizerai/grpc,fuchsia-mirror/third_party-grpc,apolcyn/grpc,adelez/grpc,madongfly/grpc,infinit/grpc,doubi-workshop/grpc,wangyikai/grpc,jboeuf/grpc,soltanmm/grpc,thunderboltsid/grpc,LuminateWireless/grpc,vsco/grpc,perumaalgoog/grpc,dklempner/grpc,greasypizza/grpc,leifurhauks/grpc,andrewpollock/grpc,muxi/grpc,baylabs/grpc,hstefan/grpc,Crevil/grpc,cgvarela/grpc,yugui/grpc,doubi-workshop/grpc,jcanizales/grpc,grani/grpc,matt-kwong/grpc,nicolasnoble/grpc,jtattermusch/grpc,MakMukhi/grpc,bjori/grpc,wcevans/grpc,sreecha/grpc,malexzx/grpc,doubi-workshop/grpc,ejona86/grpc,VcamX/grpc,tamihiro/grpc,w4-sjcho/grpc,makdharma/grpc,Crevil/grpc,bjori/grpc,a-veitch/grpc,jboeuf/grpc,stanley-cheung/grpc,miselin/grpc,bjori/grpc,madongfly/grpc,pmarks-net/grpc,ctiller/grpc,pszemus/grpc,chrisdunelm/grpc,adelez/grpc,ananthonline/grpc,quizlet/grpc,mehrdada/grpc,zhimingxie/grpc,bogdandrutu/grpc,jtattermusch/grpc,adelez/grpc,hstefan/grpc,nicolasnoble/grpc,fuchsia-mirror/third_party-grpc,yinsu/grpc,yang-g/grpc,malexzx/grpc,stanley-cheung/grpc,kriswuollett/grpc,kriswuollett/grpc,dklempner/grpc,kumaralokgithub/grpc,JoeWoo/grpc,larsonmpdx/grpc,maxwell-demon/grpc,baylabs/grpc,a-veitch/grpc,apolcyn/grpc,vjpai/grpc,sreecha/grpc,chrisdunelm/grpc,mehrdada/grpc,baylabs/grpc,pmarks-net/grpc,Vizerai/grpc,perumaalgoog/grpc,ofrobots/grpc,ncteisen/grpc,vsco/grpc,larsonmpdx/grpc,yinsu/grpc,tengyifei/grpc,vsco/grpc,ipylypiv/grpc,msiedlarek/grpc,muxi/grpc,msiedlarek/grpc,geffzhang/grpc,daniel-j-born/grpc,arkmaxim/grpc,thunderboltsid/grpc,geffzhang/grpc,andrewpollock/grpc,arkmaxim/grpc,ananthonline/grpc,vjpai/grpc,matt-kwong/grpc,makdharma/grpc,kumaralokgithub/grpc,JoeWoo/grpc,PeterFaiman/ruby-grpc-minimal,adelez/grpc,msmania/grpc,andrewpollock/grpc,ejona86/grpc,quizlet/grpc,thinkerou/grpc,chrisdunelm/grpc,kumaralokgithub/grpc,dgquintas/grpc,simonkuang/grpc,philcleveland/grpc,doubi-workshop/grpc,maxwell-demon/grpc,vjpai/grpc,nicolasnoble/grpc,rjshade/grpc,thinkerou/grpc,carl-mastrangelo/grpc,jcanizales/grpc,jtattermusch/grpc,Crevil/grpc,cgvarela/grpc,thinkerou/grpc,leifurhauks/grpc,jtattermusch/grpc,bogdandrutu/grpc,deepaklukose/grpc,tamihiro/grpc,jtattermusch/grpc,grpc/grpc,wcevans/grpc,arkmaxim/grpc,7anner/grpc,murgatroid99/grpc,quizlet/grpc,yongni/grpc,royalharsh/grpc,tengyifei/grpc,MakMukhi/grpc,soltanmm-google/grpc,ncteisen/grpc,wangyikai/grpc,bjori/grpc,greasypizza/grpc,vjpai/grpc,w4-sjcho/grpc,leifurhauks/grpc,rjshade/grpc,msmania/grpc,maxwell-demon/grpc,stanley-cheung/grpc,y-zeng/grpc,rjshade/grpc,thunderboltsid/grpc,grpc/grpc,muxi/grpc,yinsu/grpc,adelez/grpc,malexzx/grpc,sreecha/grpc,thunderboltsid/grpc,dklempner/grpc,y-zeng/grpc,ctiller/grpc,goldenbull/grpc,ctiller/grpc,perumaalgoog/grpc,ctiller/grpc,wangyikai/grpc,firebase/grpc,firebase/grpc,vsco/grpc,ncteisen/grpc,geffzhang/grpc,pmarks-net/grpc,tengyifei/grpc,MakMukhi/grpc,pszemus/grpc,pszemus/grpc,vjpai/grpc,pszemus/grpc,chrisdunelm/grpc,bjori/grpc,ananthonline/grpc,daniel-j-born/grpc,Crevil/grpc,msmania/grpc,maxwell-demon/grpc,pszemus/grpc,msmania/grpc,miselin/grpc,a-veitch/grpc,muxi/grpc,Vizerai/grpc,kpayson64/grpc,kriswuollett/grpc,yang-g/grpc,Vizerai/grpc,ananthonline/grpc,donnadionne/grpc,tamihiro/grpc,PeterFaiman/ruby-grpc-minimal,yugui/grpc,kpayson64/grpc,ananthonline/grpc,zhimingxie/grpc,ctiller/grpc,royalharsh/grpc,kumaralokgithub/grpc,kriswuollett/grpc,vjpai/grpc,muxi/grpc,nicolasnoble/grpc,yinsu/grpc,tamihiro/grpc,carl-mastrangelo/grpc,yang-g/grpc,mehrdada/grpc,stanley-cheung/grpc,yinsu/grpc,jtattermusch/grpc,VcamX/grpc,msmania/grpc,firebase/grpc,a-veitch/grpc,vjpai/grpc,LuminateWireless/grpc,kskalski/grpc,ipylypiv/grpc,deepaklukose/grpc,perumaalgoog/grpc,vjpai/grpc,stanley-cheung/grpc,7anner/grpc,royalharsh/grpc,chrisdunelm/grpc,zhimingxie/grpc,mehrdada/grpc,ctiller/grpc,greasypizza/grpc,thinkerou/grpc,yongni/grpc,deepaklukose/grpc,matt-kwong/grpc,bogdandrutu/grpc,ejona86/grpc,thunderboltsid/grpc,dgquintas/grpc,tamihiro/grpc,deepaklukose/grpc,zhimingxie/grpc,stanley-cheung/grpc,ofrobots/grpc,grpc/grpc,soltanmm-google/grpc,donnadionne/grpc,cgvarela/grpc,soltanmm-google/grpc,philcleveland/grpc,dklempner/grpc,JoeWoo/grpc,yinsu/grpc,grpc/grpc,w4-sjcho/grpc,ctiller/grpc,zhimingxie/grpc,kpayson64/grpc,simonkuang/grpc,greasypizza/grpc,daniel-j-born/grpc,makdharma/grpc,jtattermusch/grpc,yongni/grpc,grpc/grpc,ipylypiv/grpc,sreecha/grpc,ppietrasa/grpc,murgatroid99/grpc,zhimingxie/grpc,madongfly/grpc,a-veitch/grpc,simonkuang/grpc,tengyifei/grpc,fuchsia-mirror/third_party-grpc,kumaralokgithub/grpc,geffzhang/grpc,kpayson64/grpc,ejona86/grpc,Vizerai/grpc,daniel-j-born/grpc,firebase/grpc,greasypizza/grpc,andrewpollock/grpc,donnadionne/grpc,madongfly/grpc,ipylypiv/grpc,mehrdada/grpc,dgquintas/grpc,makdharma/grpc,y-zeng/grpc,greasypizza/grpc,grpc/grpc,stanley-cheung/grpc,JoeWoo/grpc,PeterFaiman/ruby-grpc-minimal,ppietrasa/grpc,pmarks-net/grpc,VcamX/grpc,murgatroid99/grpc,perumaalgoog/grpc,dgquintas/grpc,ipylypiv/grpc,yugui/grpc,nicolasnoble/grpc,a11r/grpc,pmarks-net/grpc,JoeWoo/grpc,cgvarela/grpc,grpc/grpc,dgquintas/grpc,royalharsh/grpc,makdharma/grpc,yinsu/grpc,y-zeng/grpc,makdharma/grpc,wcevans/grpc,y-zeng/grpc,Vizerai/grpc,ctiller/grpc,wcevans/grpc,Crevil/grpc,msmania/grpc,philcleveland/grpc,firebase/grpc,larsonmpdx/grpc,ananthonline/grpc,firebase/grpc,infinit/grpc,dgquintas/grpc,sreecha/grpc,bjori/grpc,yang-g/grpc,wangyikai/grpc,doubi-workshop/grpc,dgquintas/grpc,thunderboltsid/grpc,wangyikai/grpc,soltanmm-google/grpc,Crevil/grpc,simonkuang/grpc,tengyifei/grpc,carl-mastrangelo/grpc,miselin/grpc,madongfly/grpc,grpc/grpc,kumaralokgithub/grpc,rjshade/grpc,muxi/grpc,yongni/grpc,thinkerou/grpc,carl-mastrangelo/grpc,ncteisen/grpc,donnadionne/grpc,soltanmm/grpc,mehrdada/grpc,deepaklukose/grpc,hstefan/grpc,mehrdada/grpc,Crevil/grpc,nicolasnoble/grpc,tamihiro/grpc,doubi-workshop/grpc,vjpai/grpc,mehrdada/grpc,maxwell-demon/grpc,larsonmpdx/grpc,y-zeng/grpc,yugui/grpc,malexzx/grpc,malexzx/grpc,donnadionne/grpc,malexzx/grpc,deepaklukose/grpc,PeterFaiman/ruby-grpc-minimal,philcleveland/grpc,ofrobots/grpc,msmania/grpc,royalharsh/grpc,ejona86/grpc,bogdandrutu/grpc,7anner/grpc,arkmaxim/grpc,ppietrasa/grpc,infinit/grpc,ofrobots/grpc,VcamX/grpc,adelez/grpc,ejona86/grpc,ppietrasa/grpc,hstefan/grpc,daniel-j-born/grpc,nicolasnoble/grpc,leifurhauks/grpc,PeterFaiman/ruby-grpc-minimal,yongni/grpc,carl-mastrangelo/grpc,sreecha/grpc,LuminateWireless/grpc,ejona86/grpc,royalharsh/grpc,bogdandrutu/grpc,carl-mastrangelo/grpc,arkmaxim/grpc,matt-kwong/grpc,yugui/grpc,quizlet/grpc,7anner/grpc,PeterFaiman/ruby-grpc-minimal,7anner/grpc,grpc/grpc,jboeuf/grpc,quizlet/grpc,simonkuang/grpc,jboeuf/grpc,a11r/grpc,ncteisen/grpc,goldenbull/grpc,kskalski/grpc,doubi-workshop/grpc,msiedlarek/grpc,msmania/grpc,w4-sjcho/grpc,yinsu/grpc,w4-sjcho/grpc,donnadionne/grpc,stanley-cheung/grpc,grani/grpc,larsonmpdx/grpc,PeterFaiman/ruby-grpc-minimal,ppietrasa/grpc,Crevil/grpc,yugui/grpc,ctiller/grpc,mehrdada/grpc,VcamX/grpc,nicolasnoble/grpc,rjshade/grpc,kriswuollett/grpc,grani/grpc,bjori/grpc,JoeWoo/grpc,andrewpollock/grpc,muxi/grpc,podsvirov/grpc,wcevans/grpc,a11r/grpc,7anner/grpc