Improve beatmap track disposal logic

- [ ] Depends on https://github.com/ppy/osu-framework/pull/1035

osu.Game/Beatmaps/WorkingBeatmap.cs

@@ -109,10 +109,17 @@ namespace osu.Game.Beatmaps
 
         public virtual void Dispose()
         {
-            track?.Dispose();
-            track = null;
             background?.Dispose();
             background = null;
         }
+
+        public void DisposeTrack()
+        {
+            lock (trackLock)
+            {
+                track?.Dispose();
+                track = null;
+            }
+        }
     }
 }

osu.Game/OsuGameBase.cs

@@ -152,10 +152,19 @@ namespace osu.Game
 
             Beatmap.ValueChanged += b =>
             {
+                var trackLoaded = lastBeatmap?.TrackLoaded ?? false;
+
                 // compare to last beatmap as sometimes the two may share a track representation (optimisation, see WorkingBeatmap.TransferTo)
-                if (lastBeatmap?.Track != b.Track)
+                if (!trackLoaded || lastBeatmap?.Track != b.Track)
                 {
-                    lastBeatmap?.Track?.Dispose();
+                    if (trackLoaded)
+                    {
+                        Debug.Assert(lastBeatmap != null);
+                        Debug.Assert(lastBeatmap.Track != null);
+
+                        lastBeatmap.DisposeTrack();
+                    }
+
                     Audio.Track.AddItem(b.Track);
                 }