sem_timedwait with a null timeout doesn't mean "forever".

It actually means "crash immediately". Well, it's an error. And callers are
much more likely to realize their mistake if we crash immediately rather
than return EINVAL. Historically, glibc has crashed and bionic -- before
the recent changes -- returned EINVAL, so this is a behavior change.

Change-Id: I0c2373a6703b20b8a97aacc1e66368a5885e8c51

tests/semaphore_test.cpp

@@ -131,6 +131,13 @@ TEST(semaphore, sem_timedwait) {
   ASSERT_EQ(0, sem_destroy(&s));
 }
 
+TEST(semaphore_DeathTest, sem_timedwait_null_timeout) {
+  sem_t s;
+  ASSERT_EQ(0, sem_init(&s, 0, 0));
+
+  ASSERT_EXIT(sem_timedwait(&s, nullptr), testing::KilledBySignal(SIGSEGV), "");
+}
+
 TEST(semaphore, sem_getvalue) {
   sem_t s;
   ASSERT_EQ(0, sem_init(&s, 0, 0));