runtime: handle sysReserve failure in mHeap_SysAlloc
runtime: handle sysReserve failure in mHeap_SysAlloc

sysReserve will return nil on failure - correctly handle this case and return
nil to the caller. Currently, a failure will result in h.arena_end being set
to psize, h.arena_used being set to zero and fun times ensue.

On the openbsd/arm builder this has resulted in:

  runtime: address space conflict: map(0x0) = 0x40946000
  fatal error: runtime: address space conflict

When it should be reporting out of memory instead.

Change-Id: Iba828d5ee48ee1946de75eba409e0cfb04f089d4
Reviewed-on: https://go-review.googlesource.com/15056
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

dterei/go,odeke-em/go,dterei/go,vsekhar/elastic-go,derekmarcotte/go,linux-on-ibm-z/go,kim-racktop/go,nawawi/go,Cofyc/go,codestation/go,binarycrusader/go,binarycrusader/go,goodlang/good,cloudflare/go,golang/go,vanloswang/go,sndnvaps/go,akutz/go,Cofyc/go,momchil-velikov/go,vsekhar/elastic-go,theif519/go,polyverse-security/go,binarycrusader/go,CAFxX/go,cloudflare/go,kim-racktop/go,rachtsingh/gogojuice,CAFxX/go,fjballest/golang,Jordanzuo/go,akutz/go,jacobsa/go,golang/go,goodlang/good,josharian/go,Endika/go,polyverse-security/go,suncycheng/go,goodlang/good,derekmarcotte/go,nawawi/go,binarycrusader/go,roth1002/go,fjballest/golang,dterei/go,goodlang/good,akutz/go,deft-code/go,josharian/go,sndnvaps/go,4ad/go,theif519/go,vanloswang/go,4ad/go,Samurais/go,rachtsingh/gogojuice,Samurais/go,sndnvaps/go,akutz/go,gdey/go,CAFxX/go,miolini/go,CAFxX/go,momchil-velikov/go,polyverse-security/go,Cofyc/go,odeke-em/go,roth1002/go,momchil-velikov/go,polyverse-security/go,mwhudson/go,ganboing/go-esx,akutz/go,jacobsa/go,CAFxX/go,suncycheng/go,cloudflare/go,kim-racktop/go,vanloswang/go,Jordanzuo/go,odeke-em/go,ImJasonH/go,gdey/go,kim-racktop/go,ganboing/go-esx,Endika/go,binarycrusader/go,ganboing/go-esx,ganboing/go-esx,cyberroadie/go,roth1002/go,polyverse-security/go,CAFxX/go,nawawi/go,nawawi/go,rachtsingh/gogojuice,goodlang/good,mwhudson/go,miolini/go,cloudflare/go,Cofyc/go,codestation/go,mwhudson/go,cloudflare/go,cyberroadie/go,deft-code/go,binarycrusader/go,miolini/go,binarycrusader/go,suncycheng/go,odeke-em/go,odeke-em/go,codestation/go,theif519/go,malvira/go,mwhudson/go,akutz/go,kim-racktop/go,linux-on-ibm-z/go,goodlang/good,goodlang/good,malvira/go,dterei/go,malvira/go,kim-racktop/go,roth1002/go,linux-on-ibm-z/go,4ad/go,rachtsingh/gogojuice,josharian/go,nawawi/go,vsekhar/elastic-go,suncycheng/go,Samurais/go,fjballest/golang,golang/go,Endika/go,fjballest/golang,deft-code/go,theif519/go,akutz/go,4ad/go,malvira/go,miolini/go,josharian/go,cyberroadie/go,Samurais/go,deft-code/go,golang/go,linux-on-ibm-z/go,Samurais/go,cloudflare/go,Cofyc/go,miolini/go,4ad/go,josharian/go,dterei/go,4ad/go,suncycheng/go,ImJasonH/go,josharian/go,fjballest/golang,rachtsingh/gogojuice,christopher-henderson/Go,Jordanzuo/go,polyverse-security/go,nawawi/go,theif519/go,vanloswang/go,malvira/go,malvira/go,jacobsa/go,sndnvaps/go,ganboing/go-esx,gdey/go,Endika/go,gdey/go,momchil-velikov/go,jacobsa/go,vanloswang/go,sndnvaps/go,jacobsa/go,Endika/go,Jordanzuo/go,codestation/go,roth1002/go,golang/go,Samurais/go,vanloswang/go,gdey/go,roth1002/go,malvira/go,deft-code/go,polyverse-security/go,derekmarcotte/go,vsekhar/elastic-go,Cofyc/go,golang/go,mwhudson/go,Cofyc/go,rachtsingh/gogojuice,Endika/go,suncycheng/go,gdey/go,dterei/go,Jordanzuo/go,ImJasonH/go,kim-racktop/go,ImJasonH/go,theif519/go,cyberroadie/go,cyberroadie/go,ganboing/go-esx,dterei/go,odeke-em/go,miolini/go,deft-code/go,theif519/go,codestation/go,odeke-em/go,linux-on-ibm-z/go,deft-code/go,vsekhar/elastic-go,Endika/go,rachtsingh/gogojuice,miolini/go,odeke-em/go,4ad/go,momchil-velikov/go,kim-racktop/go,cyberroadie/go,vanloswang/go,ganboing/go-esx,suncycheng/go,rachtsingh/gogojuice,CAFxX/go,fjballest/golang,gdey/go,gdey/go,momchil-velikov/go,cyberroadie/go,codestation/go,malvira/go,fjballest/golang,mwhudson/go,ImJasonH/go,Jordanzuo/go,codestation/go,derekmarcotte/go,derekmarcotte/go,dterei/go,cloudflare/go,vsekhar/elastic-go,codestation/go,cyberroadie/go,jacobsa/go,Endika/go,Cofyc/go,binarycrusader/go,deft-code/go,golang/go,Samurais/go,fjballest/golang,momchil-velikov/go,linux-on-ibm-z/go,nawawi/go,derekmarcotte/go,polyverse-security/go,Jordanzuo/go,golang/go,derekmarcotte/go,cloudflare/go,momchil-velikov/go,akutz/go,goodlang/good,ImJasonH/go,ImJasonH/go,nawawi/go,josharian/go,linux-on-ibm-z/go,jacobsa/go,mwhudson/go,Samurais/go,4ad/go,Jordanzuo/go,miolini/go,roth1002/go,roth1002/go,ImJasonH/go,josharian/go,CAFxX/go,sndnvaps/go,mwhudson/go,derekmarcotte/go,sndnvaps/go,jacobsa/go,vanloswang/go,theif519/go,suncycheng/go,vsekhar/elastic-go,linux-on-ibm-z/go,sndnvaps/go,vsekhar/elastic-go,ganboing/go-esx