Fix for large records with more than 2^16 bytes data blocks (XXXX-record handling)

This fix is the solution for issue #33 (the XXXX record showing up in the latest USKP header cannot be parsed) and should be a general fix, i.e. also work for other records with a prepended XXXX.

Please note that this is not yet carefully tested, I and one of my beta testers gave it a quick try, but due to the connection problems my Internet Service provider has at present, I decided to create this pull request as soon as possible, to avoid duplicate work.

In parallel to this pull request, I have published an experimental version of the Reqtificator and send out a call for testing to the Requiem users to gather more feedback.