OpenSystemsDevelopment / qpdf

18.04 is out of security support, and cmake for 18.04 is no longer
available via kitware's debian repository (or at least fetching it
failed for one build).

It is possible to reproduce the failure with this file following the
instructions with oss-fuzz, though it does not cause a failure in CI.

The failure was introduced in
18c1ffe0df335a46cddbeb96e2cb939d850df9fa.

Prevent destruction of shared null objects

#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.

Fix bug in QPDF_Name::normalizeName introduced in #891

'#' was omitted from the list of chars that need to be escaped.

C-API : expose QPDFJob::createQPDF and writeQPDF

Also, move _qpdf_data and to new header filer _qpdf_error to new header
file qpdf-c_impl.hh

Refactor QPDF_Array

Code tidy re-throwing of exceptions

Tune indentations in QPDFWriter::unparseObject

Refactor QUtil::hex_decode