Makes PDF files load when xrefEntry is undefined

Fixes http://www.prensalibre.com/multimedia/Terremoto-1976-Sismo-terremoto_del_76-guatemala-Teremoto-temblorgt_PREFIL20140203_0001.pdf from http://bthorben.github.io/pdfRepo.

Thanks to @bthorben and the Opera team for listing problematic PDFs at http://bthorben.github.io/pdfRepo!