Don't allow to drag'n'drop between databases to fix a crash

git-svn-id: https://svn.code.sf.net/p/keepassx/code/trunk@373 b624d157-de02-0410-bad0-e51aec6abb33

src/lib/GroupView.cpp

@@ -206,6 +206,9 @@ void KeepassGroupView::dragEnterEvent ( QDragEnterEvent * event ){
 	LastHoverItem=NULL;
 	InsLinePos=-1;
 	
+	if (event->source() == NULL)
+		return; // drag event came from another application
+	
 	if(event->mimeData()->hasFormat("application/x-keepassx-group")){
 		DragType=GroupDrag;
 		event->acceptProposedAction();