cmd/gc: fix heap buffer overflow Fixes issue 8528.
cmd/gc: fix heap buffer overflow
Fixes issue 8528.

LGTM=rsc
R=rsc, r, iant, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/128230045

wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang