Fix memory leak in pdf_xref_size_from_old_trailer.

Thanks to Simon for spotting the original problem. This is a slight
tweak on the patch he supplied.
fix memory leak in pdf_repair_xref

The 0 null object is leaked if a document refers to 0 0 obj before
requiring a delayed reparation (seen e.g. with 3324.pdf.asan.3.2585).