dispatch: fix a race condition triggering an assert()

If we were waiting, and then exiting due to timeout, we still have to
recheck the condition protected by the condition variable/mutex in order
to get back to a consistent state. In this case, the queue was locked
with mp_dispatch_lock(), and mp_dispatch_queue_process() got to return
without waiting for unlock.

Also caused commit 8716c2e. Probably an argument for replacing the
dispatch queue by a simple mutex.

misc/dispatch.c

@@ -249,7 +249,7 @@ void mp_dispatch_queue_process(struct mp_dispatch_queue *queue, double timeout)
         } else if (wait > 0 && !queue->interrupted) {
             struct timespec ts = mp_time_us_to_timespec(wait);
             if (pthread_cond_timedwait(&queue->cond, &queue->lock, &ts))
-                break;
+                wait = 0;
         } else {
             break;
         }