Playlist: randomize next song when enabling "random" mode while not playing

Don't restore the current song after shufflung when MPD is stopped
(but still remembers the current song internally).  Fixes the first
part of Mantis ticket 0004005.

src/Playlist.cxx

@@ -294,7 +294,9 @@ playlist::SetRandom(PlayerControl &pc, bool status)
 	if (queue.random) {
 		/* shuffle the queue order, but preserve current */
 
-		const int current_position = GetCurrentPosition();
+		const int current_position = playing
+			? GetCurrentPosition()
+			: -1;
 
 		queue.ShuffleOrder();