playlist: don't shuffle if not in random mode

One of the previous patches removed the "random" mode check from
nextSongInPlaylist(), which caused a shuffle whenever MPD wrapped to
the first song in "repeat" mode.  Re-add that "random" check.

src/playlist.c

@@ -772,7 +772,7 @@ void nextSongInPlaylist(void)
 		return;
 	}
 
-	if (next_order == 0) {
+	if (next_order == 0 && playlist.queue.random) {
 		assert(playlist.queue.repeat);
 
 		queue_shuffle_order(&playlist.queue);