Fix assert showing default-constructed QPrintPreviewDialog

Fix call to qBound(), avoiding an assert introduced
by ad5c5bb541.

Pick-to: 6.3
Fixes: QTBUG-101297
Change-Id: I823cdfcd2491c6bb4c87058318479d188ca38742
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/printsupport/widgets/qprintpreviewwidget.cpp

@@ -401,7 +401,7 @@ void QPrintPreviewWidgetPrivate::generatePreview()
     pictures = printer->d_func()->previewPages();
     populateScene(); // i.e. setPreviewPrintedPictures() e.l.
     layoutPages();
-    curPage = qBound(1, curPage, pages.count());
+    curPage = pages.count() > 0 ? qBound(1, curPage, pages.count()) : 1;
     if (fitting)
         _q_fit();
     emit q->previewChanged();