avoid crash on stupid but allowable usage of pthread_mutex_unlock

unlocking an unlocked mutex is not UB for robust or error-checking
mutexes, so we must avoid calling __pthread_self (which might crash
due to lack of thread-register initialization) until after checking
that the mutex is locked.

src/thread/pthread_mutex_unlock.c

@@ -5,6 +5,8 @@ int pthread_mutex_unlock(pthread_mutex_t *m)
 	pthread_t self;
 
 	if (m->_m_type != PTHREAD_MUTEX_NORMAL) {
+		if (!m->_m_lock)
+			return EPERM;
 		self = __pthread_self();
 		if ((m->_m_lock&0x1fffffff) != self->tid)
 			return EPERM;