OpenSystemsDevelopment / qpdf

Thanks to github user zdenop for supplying some additional
error-handling code.

Use PointerHolder in several places where manually memory allocation
and deallocation were being used. This helps to protect against memory
leaks when exceptions are thrown in surprising places.

This code was essentially duplicated between test_driver and
standalone_fuzz_target_runner.

In a small number of cases, it makes sense to replace an overloaded
function with a function that takes a default argument. We can do this
now because we've already broken binary compatibility since the last
release.

Clang seems to be slightly stricter than gcc.

Have classes contain only a single private member of type
PointerHolder<Members>. This makes it safe to change the structure of
the Members class without breaking binary compatibility. Many of the
classes already follow this pattern quite successfully. This brings in
the rest of the class that are part of the public API.

I don't want to maintain them or make anyone think they should be used.

* Several assertions in linearization were not always true; change
  them to run time errors
* Handle a few cases of uninitialized objects
* Handle pages with no contents when doing form operations
* Handle invalid page tree nodes when traversing pages

* Create the seed corpus in the build directory
* Don't assume all fuzzers share an options file

Now that there aren't any more...

This fixes cases of warning C4800 from msvc

This makes all integer type conversions that have potential data loss
explicit with calls that do range checks and raise an exception. After
this commit, qpdf builds with no warnings when -Wsign-conversion
-Wconversion is used with gcc or clang or when -W3 -Wd4800 is used
with MSVC. This significantly reduces the likelihood of potential
crashes from bogus integer values.

There are some parts of the code that take int when they should take
size_t or an offset. Such places would make qpdf not support files
with more than 2^31 of something that usually wouldn't be so large. In
the event that such a file shows up and is valid, at least qpdf would
raise an error in the right spot so the issue could be legitimately
addressed rather than failing in some weird way because of a silent
overflow condition.

This is the type we need for the underlying zlib implementation.

Change from unsigned long to int since we pass enumerated type values
to this field.