OpenSystemsDevelopment / qpdf

Add new convenience class QPDFObjGen::Guard

#863 uses a single null object for nulls that were previously implicit. In
certain circumstances this shared null object gets destroyed (i.e changed
to a QPDF_Destroyed object) when a QPDF object is destroyed.

Modify the QPDF destructor so that null objects get disconnected from the
dying QPDF object but not destroyed to prevent this from happening.

Code tidy re-throwing of exceptions

Avoid copying exceptions.

A new private overload of QPDF::makeIndirectObject breaks pikepdf's
build, so renaming function.

An indirect object reference to 0, 0 is invalid. If it appears in the
file or is parsed from a string, the parser catches it. This check
would only be useful for someone explicitly calling getObject with 0,
0, and that would trigger an error during resolve().