Fix crash if adding sublayers via plugin interface

src/app/qgisapp.cpp

@@ -7322,7 +7322,7 @@ QgsVectorLayer* QgisApp::addVectorLayer( QString vectorLayerPath, QString baseNa
       // The first layer loaded is not useful in that case. The user can select it in
       // the list if he wants to load it.
       delete layer;
-
+      layer = 0;
     }
     else
     {