Fix pthread_create error handling in ril harder.

The previous patch fixed the behavior _if_ something goes wrong...
...but didn't fix the check for whether something went wrong.
pthread_create isn't a regular Unix "return -1 and set errno" kind
of function; it returns 0 on success and an error number (without
setting errno) on failure.

Bug: https://code.google.com/p/android/issues/detail?id=64189
Change-Id: I335af890885d6a672d7bac746fdfea3ed3995360