Add precautionary assertion guarding against song select messing with logo while not current

I considered having this just be a straight guard followed by an
early-return, but if that guard ever actually gets hit, things are going
to be severely broken all over, and un-breaking them at that point will
be very annoying, so just going to cross fingers and hope this can be an
assertion forevermore instead.

osu.Game/Screens/SelectV2/SongSelect.cs

@@ -278,6 +278,8 @@ namespace osu.Game.Screens.SelectV2
 
             modSelectOverlay.State.BindValueChanged(v =>
             {
+                Debug.Assert(this.IsCurrentScreen());
+
                 logo?.ScaleTo(v.NewValue == Visibility.Visible ? 0f : logo_scale, 400, Easing.OutQuint)
                     .FadeTo(v.NewValue == Visibility.Visible ? 0f : 1f, 200, Easing.OutQuint);
             });