runtime: stack copier should handle nil defers without faulting.
runtime: stack copier should handle nil defers without faulting.

fixes issue 8047

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://codereview.appspot.com/101800043

tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme