QPdfDocumentPrivate::load(): check the first two pages

This seems to make QPdfLinkModel happy for some reason; otherwise
QPdfLinkModelPrivate::update() does not succeed in getting page numbers
when it calls FPDFDest_GetDestPageIndex().
Amends 9231d344



Change-Id: Iaed3301a1ab304ac9813c3b605b2f9c7465bf8e7
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>