resolved: packet - fail on invalid zero-length data

resolved: packet - fail on invalid zero-length data

Most blobs (keys, signatures, ...) should have a specific size given by
the relevant algorithm. However, as we don't use/verify the algorithms
yet, let's just ensure that we don't read out zero-length data in cases
where this does not make sense.

The only exceptions, where zero-length data is allowed are in the NSEC3
salt field, and the generic data (which we don't know anything about,
so  better not make any assumptions).