QWidget: init data in ctor, not Private::init()

This should fix Coverity's hallucination that QWidget::data is used
uninitialized, or, after 58b9250aea0b1b41c8cbd1033149371a93a5b2c1,
dereferenced while it's still nullptr.

Coverity-Id: 475552
Coverity-Id: 475551
Coverity-Id: 475550
Coverity-Id: 475549
Coverity-Id: 396828
Pick-to: 6.9 6.8 6.5
Change-Id: I49705b8bc44082fc68a59c2c98c4e37a6d841b19
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -852,6 +852,7 @@ QWidget::QWidget(QWidget *parent, Qt::WindowFlags f)
 */
 QWidget::QWidget(QWidgetPrivate &dd, QWidget* parent, Qt::WindowFlags f)
     : QObject(dd, nullptr), QPaintDevice()
+    , data(&dd.data)
 {
     Q_D(QWidget);
     QT_TRY {
@@ -935,8 +936,6 @@ void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
     if (allWidgets)
         allWidgets->insert(q);
 
-    q->data = &data;
-
 #if QT_CONFIG(thread)
     if (!parent) {
         Q_ASSERT_X(QThread::isMainThread(), "QWidget",