decoder: remove incorrect assertion

The decoder can be paused by the decoder owner while the decoder thread
is decoding. We still need to queue the last decoded picture(s) to the
(not yet paused) video output.

src/input/decoder.c

@@ -906,13 +906,10 @@ static void DecoderPlayVideo( decoder_t *p_dec, picture_t *p_picture,
     vlc_mutex_unlock( &p_owner->lock );
 
     /* FIXME: The *input* FIFO should not be locked here. This will not work
-     * properly if/when pictures are queued asynchronously (c.f. assert()). */
+     * properly if/when pictures are queued asynchronously. */
     vlc_fifo_Lock( p_owner->p_fifo );
-    if( p_owner->paused )
-    {
-       assert( p_owner->frames_countdown > 0 );
-       p_owner->frames_countdown--;
-    }
+    if( unlikely(p_owner->paused) && likely(p_owner->frames_countdown > 0) )
+        p_owner->frames_countdown--;
     vlc_fifo_Unlock( p_owner->p_fifo );
 
     /* */