LIB-16: test for invalid track lengths

On copy protected there are sometimes (extra) tracks that are no valid
audio tracks, but also not marked (in control) as data tracks.
Some of these have an invalid track length so we do know they are wrong.

This doesn't fix all problems.
don't leave old offsets in the structure

If we change last_audio_track we should also set old offsets to 0.
These are not part of the url, but part of the ID calculation.