pthread_rwlock_unlock(3) returns 0 if successful, otherwise an error number
will be returned to indicate the error, so I'm applying an assert(3) to do
a sanity check of the return value.

Reported by: Coverity CID: 1391235, 1193654 and 1193651
Reviewed by: grehan
MFC after: 4 weeks.
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D15533