player: wake up decoder before waiting for xfade chunks

Fix a deadlock: when the decoder waited for buffer space, the player
could enter a deadlock situation because it waits for more chunks for
crossfading chunks.  Signal the decoder before entering notify_wait().

src/player_thread.c

@@ -429,6 +429,7 @@ static void do_play(void)
 						/* wait for the
 						   decoder */
 						music_pipe_set_lazy(false);
+						notify_signal(&dc.notify);
 						notify_wait(&pc.notify);
 						continue;
 					}