misc/cgo/test: fix bad C test code that fails on some configurations
misc/cgo/test: fix bad C test code that fails on some configurations

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/169800043

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