cgo: accept null pointers in gccgo flavour of C.GoString.
cgo: accept null pointers in gccgo flavour of C.GoString.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5569074
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go