Fix crash in QNetworkSession when engine is not set

Task-number: QTBUG-42427
Change-Id: Ie35d5e0f72be3a16ecb6b928e12bc50d9f183590
Reviewed-by: Andy Shaw <andy.shaw@digia.com>

src/plugins/bearer/qnetworksession_impl.cpp

@@ -366,7 +366,8 @@ void QNetworkSessionPrivateImpl::networkConfigurationsChanged()
     else
         updateStateFromActiveConfig();
 
-    startTime = engine->startTime(activeConfig.identifier());
+    if (engine)
+        startTime = engine->startTime(activeConfig.identifier());
 }
 
 void QNetworkSessionPrivateImpl::configurationChanged(QNetworkConfigurationPrivatePointer config)