Fix crash when calling QMainWindow::restoreState()

Restoring the state of a window with a dock widget could cause a crash
in a multiple display configuration if the scale or screen arrangement
in the system had changed after the state was saved. In this case, the
original top-left coordinate of the window could now lie outside of any
screens, which would result in a NULL QScreen pointer being dereferenced
inside QDockAreaLayout::constrainedRect().

Fixes: QTBUG-102718
Fixes: QTBUG-102541
Pick-to: 6.2 6.3
Change-Id: Ie5e3209b82a33a1dc4568611c1b2ee3a6d8830b7
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/widgets/qdockarealayout.cpp

@@ -3018,10 +3018,10 @@ QSize QDockAreaLayout::minimumStableSize() const
  */
 QRect QDockAreaLayout::constrainedRect(QRect rect, QWidget* widget)
 {
-    QScreen *screen;
+    QScreen *screen = nullptr;
     if (QGuiApplication::primaryScreen()->virtualSiblings().size() > 1)
         screen = QGuiApplication::screenAt(rect.topLeft());
-    else
+    if (!screen)
         screen = widget->screen();
 
     const QRect screenRect = screen->geometry();