QWizard: Fix null pointer dereference if no pixmap was set

The pixmap may be set in QWizardPrivate::updateLayout() if there's a
side widget or a watermark. If neither exists, then no pixmap is set and
this would cause a crash.

Task-number: QTBUG-61423
Change-Id: Ia53158e207a94bf49489fffd14c80dd93415dd0f
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/widgets/dialogs/qwizard.cpp

@@ -450,7 +450,7 @@ public:
     }
 
     QSize minimumSizeHint() const Q_DECL_OVERRIDE {
-        if (!pixmap() && !pixmap()->isNull())
+        if (pixmap() && !pixmap()->isNull())
             return pixmap()->size();
         return QFrame::minimumSizeHint();
     }