There were a few places in the code that were checking that a pointer
wasn't null before deleting it, even though C++ has always allowed
delete 0. Most of the code did not perform these checks.
Versions prior to 4.6 didn't allow gcc diagnostic pragmas with push
and pop and to appear anywhere in the file. Just let the warning be
there for those versions.
Significantly improve the code's use of off_t for file offsets, size_t
for memory sizes, and integer types in cases where there has to be
compatibility with external interfaces. Rework sections of the code
that would have prevented qpdf from working on files larger than 2 (or
maybe 4) GB in size.
