[release-branch.go1.1] runtime: zeroize g->fnstart to not prevent GC of the closure

««« CL 9557043 / 2c128d417029
runtime: zeroize g->fnstart to not prevent GC of the closure
Fixes #5493.

R=golang-dev, minux.ma, iant
CC=golang-dev
https://golang.org/cl/9557043
»»»

R=dvyukov, iant, minux.ma, bradfitz, dave
CC=golang-dev
https://golang.org/cl/10031043