Don't poll while in room

Fixes timeout in `TestJoinRoomWithoutPassword`, because the 'server' returns
out-of-date data while the `MatchSubScreen` has possible not been entered
yet (and thus hasn't disabled polling itself yet).

Can be tested by adding a `Task.Delay(3000);` at the end of
the `MultiplayerClient.JoinRoom()` task.

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

@@ -93,6 +93,9 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer
                 if (!isConnected.Value)
                     return Task.CompletedTask;
 
+                if (client.Room != null)
+                    return Task.CompletedTask;
+
                 return base.Poll();
             }
         }