cmd/gc: fix nil pointer dereference
cmd/gc: fix nil pointer dereference

LGTM=iant
R=golang-codereviews, dave, iant
CC=golang-codereviews
https://codereview.appspot.com/60740044

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