cmd/cgo: compare malloc return value to NULL instead of literal 0

Follow the pattern used in the other _cgoPREFIX_Cfunc* functions. This
also avoids a -Wzero-as-null-pointer-constant warning when compiling
with g++.

Change-Id: I95ac8842df048105f4c738f3603136d9cfa2dfdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/664916
Auto-Submit: Keith Randall <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: tklauser

src/cmd/cgo/out.go

@@ -1812,7 +1812,7 @@ void _cgoPREFIX_Cfunc__Cmalloc(void *v) {
 	void *ret;
 	_cgo_tsan_acquire();
 	ret = malloc(a->p0);
-	if (ret == 0 && a->p0 == 0) {
+	if (ret == NULL && a->p0 == 0) {
 		ret = malloc(1);
 	}
 	a->r1 = ret;