runtime/testdata/testprogcgo: add explicit return value to signalThread

Should fix the clang builder.

Change-Id: I3ee34581b6a7ec902420de72a8a08a2426997782
Reviewed-on: https://go-review.googlesource.com/30363
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/testdata/testprogcgo/racesig.go

@@ -31,7 +31,7 @@ static void* signalThread(void* p) {
 	for (i = 0; i < 100; i++) {
 		for (j = 0; j < ALLOCERS; j++) {
 			if (pthread_kill(pt[j], SIGCHLD) < 0) {
-				return;
+				return NULL;
 			}
 		}
 		usleep(1);