Check for selfinclusion of assets (closes #1032)

Make sure that OpenSpace doesn't crash when bailing out early

src/engine/openspaceengine.cpp

@@ -925,6 +925,8 @@ void OpenSpaceEngine::deinitializeGL() {
         func();
     }
 
+    _loadingScreen = nullptr;
+
     global::deinitializeGL();
 
     rendering::helper::deinitialize();