OpenSystemsDevelopment / qpdf

- Add null pointer checks for schema objects.
- Refactor `checkSchemaInternal` for better readability and maintainability.
- Replace redundant error construction logic with lambda-based approach.
- Simplify array and dictionary schema handling.
- Remove unused test coverage statistics.

- Standardize error handling by replacing repetitive throw statements with helper functions.
- Add test coverage for new helper functions in error handling.
- Enhance maintainability and readability with concise utilities.

If duplicate keys are encountered, overwrite earlier values with the latest
value.

Also, reject them in strings.

Also. remove '.' as  starting char in lsTop.

When the schema wants a variable-length array, allow a single item as
well as allowing an array.

This was used, but it no longer is, so let's not keep the extra
complexity around.

Avoid calling jpeg_mem_src and jpeg_mem_dest. The custom destination
manager writes to the pipeline in smaller chunks to avoid having the
whole image in memory at once. The source manager works directly with
the Buffer object. Using customer managers avoids use of memory source
and destination managers, which are not present in older versions of
libjpeg still in use by some Linux distributions.

Preparing to refactor some pattern searching code to use these instead
of their own memchr loops. This should simplify the code that replaces
PCRE.

git-svn-id: svn+q:///qpdf/trunk@599 71b93d88-0707-0410-a8cf-f5a4172ac649