Assert that QWidgetPrivate::create creates a window

Assert the expected side effect of createTLSysExtra, which might not
allocate a window, but must do so in this case (as we have already
returned if the QWidget is not a window).

Fixes static analyzer warning 2f3bbfe8addb586445e96f8906d6769e

Pick-to: 6.1
Change-Id: I4d5b8651b3510eff8e4a7b25889c0521ba6a4247
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -1246,6 +1246,7 @@ void QWidgetPrivate::create()
     // in case the extra was already valid.
     if (!win) {
         createTLSysExtra();
+        Q_ASSERT(topData()->window);
         win = topData()->window;
     }