fix potential undefined DnD behavior for external drags

dragging from the tree-view (e.g. favorite sidebar or plugin-manager)
will not set _drag_source.
drag-motion may wrongly interpret it as "re-order".

libs/gtkmm2ext/gtkmm2ext/dndvbox.h

@@ -418,6 +418,7 @@ private:
 	{
 		delete _drag_icon;
 		_drag_icon = 0;
+		_drag_source = 0;
 
 		_drag_child = 0;
 		remove_placeholder ();