Fix multiplayer match screen being exited from when not current

This was supposed to be fixed by #24255, but has popped up as a
regression on Sentry since:

	https://sentry.ppy.sh/organizations/ppy/issues/22749/?project=2&referrer=regression_activity-email

On a fifteen-minute check I cannot figure out how to reproduce, so
rather than spending further brain cycles on this, just apply the same
explicit guard that like fifteen other places do.

osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSubScreen.cs

@@ -265,7 +265,8 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
                     dialogOverlay.Push(new ConfirmDialog("Are you sure you want to leave this multiplayer match?", () =>
                     {
                         exitConfirmed = true;
-                        this.Exit();
+                        if (this.IsCurrentScreen())
+                            this.Exit();
                     }));
                 }