player_thread: check command before waiting during pause

While paused, the player thread re-locks its mutex and waits for a
signal.  This is racy: when the command is set while the thread is
waiting for the lock, it may wait forever.  This patch adds another
command check before player_wait().

src/player_thread.c

@@ -819,6 +819,8 @@ static void do_play(struct decoder_control *dc)
 
 		if (player.paused) {
 			player_lock();
+
+			if (pc.command == PLAYER_COMMAND_NONE)
 				player_wait();
 			continue;
 		} else if (music_pipe_size(player.pipe) > 0) {