Qt: PopupMenu: Stop crashing on VOut update. (fix #7016)

When calling a modal dialog, Qt switches to a new event loop.
The call may still have some event to process in its own queue,
and deleting it when doing the Vout update will have those events
refering to a no longer existing widget.

modules/gui/qt4/menus.cpp

@@ -769,7 +769,8 @@ QMenu *VLCMenuBar::HelpMenu( QWidget *parent )
  *****************************************************************************/
 #define POPUP_BOILERPLATE \
     static QMenu* menu = NULL;  \
-    delete menu; menu = NULL; \
+    if ( menu ) menu->deleteLater(); \ /* Ensure it is deleted after processing events */
+    menu = NULL; \                     /* or we might crash on an incoming mouseup event */
     if( !show ) \
         return; \
     QVector<vlc_object_t *> objects; \