fixes MusicController constantly trying to start a track

In Detail:
It tried to start a track if the current one ended even if no BeatmapSets were present.
Also if only one BeatmapSet is present if will loop by itself now.

osu.Game/Overlays/Music/PlaylistOverlay.cs

@@ -149,6 +149,12 @@ namespace osu.Game.Overlays.Music
         private void playSpecified(BeatmapInfo info)
         {
             beatmapBacking.Value = beatmaps.GetWorkingBeatmap(info, beatmapBacking);
+
+            if (BeatmapSets.Count() == 1)
+                beatmapBacking.Value.Track.Looping = true;
+            else
+                beatmapBacking.Value.Track.Looping = false;
+
             beatmapBacking.Value.Track.Start();
         }
     }

osu.Game/Overlays/MusicController.cs

@@ -251,7 +251,7 @@ namespace osu.Game.Overlays
 
                 playButton.Icon = track.IsRunning ? FontAwesome.fa_pause_circle_o : FontAwesome.fa_play_circle_o;
 
-                if (track.HasCompleted && !track.Looping && !beatmapBacking.Disabled)
+                if (track.HasCompleted && !beatmapBacking.Disabled && playlist.BeatmapSets.Any())
                     next();
             }
             else