playlist: fix "clear" crash in random mode

When MPD is not playing while in random mode, and the client issues
the "clear" command, MPD crashes in stopPlaylist(), or more exactly,
in queue_order_to_position(-1).  Exit from stopPlaylist() if MPD isn't
playing.

src/playlist.c

@@ -660,6 +660,11 @@ deleteASongFromPlaylist(const struct song *song)
 
 void stopPlaylist(void)
 {
+	if (!playlist.playing)
+		return;
+
+	assert(playlist.current >= 0);
+
 	g_debug("playlist: stop");
 	playerWait();
 	playlist.queued = -1;