cmd/gc: fix error check for self-referential array type.
cmd/gc: fix error check for self-referential array type.

LGTM=gri, iant
R=gri, iant
CC=golang-codereviews
https://codereview.appspot.com/75920044

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