audio/out/push: make draining slightly more robust

Don't wait after the audio thread has pushed the remaining audio to the
AO. Avoids hard hangs if the heuristic fails completely (could still
happen if get_delay returns absurd values).

CC: @mpv-player/stable

audio/out/push.c

@@ -151,7 +151,7 @@ static void drain(struct ao *ao)
 
     p->final_chunk = true;
     wakeup_playthread(ao);
-    while (p->still_playing)
+    while (p->still_playing && mp_audio_buffer_samples(p->buffer) > 0)
         pthread_cond_wait(&p->wakeup, &p->lock);
 
     if (ao->driver->drain) {