dispatch: wakeup only if needed on mp_dispatch_resume()

The wakeup is needed to make mp_dispatch_queue_process() return if
suspension is not needed anymore - which is only the case when the
request count reaches 0.

The assertion added with this commit always has/had to be true.

misc/dispatch.c

@@ -238,9 +238,11 @@ void mp_dispatch_suspend(struct mp_dispatch_queue *queue)
 void mp_dispatch_resume(struct mp_dispatch_queue *queue)
 {
     pthread_mutex_lock(&queue->lock);
+    assert(queue->suspended);
     assert(queue->suspend_requested > 0);
     queue->suspend_requested--;
-    pthread_cond_broadcast(&queue->cond);
+    if (queue->suspend_requested == 0)
+        pthread_cond_broadcast(&queue->cond);
     pthread_mutex_unlock(&queue->lock);
 }