Do not attempt to stop preview tracks when arriving from a "track com…

…pleted" sync

This fixes an issue identified with the WASAPI implementation in
ppy/osu-framework#6088. It has no real effect
on current `master`, but fixes a deadlock that occurs with the
aforementioned framework branch when one lets a preview track play out
to the end - at this point all audio will stop and an attempt to perform
any synchronous BASS operation (playing another track, seeking) will
result in a deadlock.

It isn't terribly clear as to why this is happening precisely, but
there does not appear to be any need to stop and seek at that point,
so this feels like a decent workaround even if the actual issue is
upstream (and will unblock pushing out WASAPI support to users).

osu.Game/Audio/PreviewTrack.cs

@@ -96,10 +96,13 @@ public void Stop()
 
             hasStarted = false;
 
-            Track.Stop();
+            if (!Track.HasCompleted)
+            {
+                Track.Stop();
 
-            // Ensure the track is reset immediately on stopping, so the next time it is started it has a correct time value.
-            Track.Seek(0);
+                // Ensure the track is reset immediately on stopping, so the next time it is started it has a correct time value.
+                Track.Seek(0);
+            }
 
             Stopped?.Invoke();
         }