QDataWidgetMapper: Fix UB (member call)

As found by UBSan:

  qdatawidgetmapper.cpp:212:59: runtime error: member call on address 0x2b6cc8095be0 which does not point to an object of type 'QFocusHelper'
  0x2b6cc8095be0: note: object is of type 'QLineEdit'

Just make QDataWidgetMapperPrivate a friend of QWidget.

Change-Id: I33d8d430c3a03b7173358d0f96dc7f850d11697c
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/itemviews/qdatawidgetmapper.cpp

@@ -199,20 +199,6 @@ void QDataWidgetMapperPrivate::_q_commitData(QWidget *w)
     commit(widgetMap.at(idx));
 }
 
-class QFocusHelper: public QWidget
-{
-public:
-    bool focusNextPrevChild(bool next) Q_DECL_OVERRIDE
-    {
-        return QWidget::focusNextPrevChild(next);
-    }
-
-    static inline void focusNextPrevChild(QWidget *w, bool next)
-    {
-        static_cast<QFocusHelper *>(w)->focusNextPrevChild(next);
-    }
-};
-
 void QDataWidgetMapperPrivate::_q_closeEditor(QWidget *w, QAbstractItemDelegate::EndEditHint hint)
 {
     int idx = findWidget(w);
@@ -224,10 +210,10 @@ void QDataWidgetMapperPrivate::_q_closeEditor(QWidget *w, QAbstractItemDelegate:
         populate(widgetMap[idx]);
         break; }
     case QAbstractItemDelegate::EditNextItem:
-        QFocusHelper::focusNextPrevChild(w, true);
+        w->focusNextChild();
         break;
     case QAbstractItemDelegate::EditPreviousItem:
-        QFocusHelper::focusNextPrevChild(w, false);
+        w->focusPreviousChild();
         break;
     case QAbstractItemDelegate::SubmitModelCache:
     case QAbstractItemDelegate::NoHint:

src/widgets/kernel/qwidget.h

@@ -668,6 +668,7 @@ protected:
     void destroy(bool destroyWindow = true,
                  bool destroySubWindows = true);
 
+    friend class QDataWidgetMapperPrivate; // for access to focusNextPrevChild
     virtual bool focusNextPrevChild(bool next);
     inline bool focusNextChild() { return focusNextPrevChild(true); }
     inline bool focusPreviousChild() { return focusNextPrevChild(false); }