Fix NetworkManager not resetting LastUpdateTime

NetworkManager did not reset the LastUpdateTime variable to 0 when
stopping. This lead to wrong behaviour in Update when Stopped and
Started multiple times.

Source/Engine/Networking/NetworkManager.cpp

@@ -364,6 +364,8 @@ void NetworkManager::Stop()
 
     State = NetworkConnectionState::Disconnected;
     Mode = NetworkManagerMode::Offline;
+    LastUpdateTime = 0;
+
     StateChanged();
 }