[asan] Add a missing return.

It is technically reachable on Android, where pthread_exit is not
marked noreturn.

llvm-svn: 150381

compiler-rt/lib/asan/tests/asan_test.cc

@@ -1643,6 +1643,7 @@ TEST(AddressSanitizer, ThreadedStressStackReuseTest) {
 
 static void *PthreadExit(void *a) {
   pthread_exit(0);
+  return 0;
 }
 
 TEST(AddressSanitizer, PthreadExitTest) {