Fix event leak in Multiplayer implementation

Very likely closes ppy#29088. It's the only thing I could find odd in the
memory dump.

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

@@ -108,7 +108,10 @@ protected override void Dispose(bool isDisposing)
             base.Dispose(isDisposing);
 
             if (client.IsNotNull())
+            {
                 client.RoomUpdated -= onRoomUpdated;
+                client.GameplayAborted -= onGameplayAborted;
+            }
         }
     }
 }