When looking for the ENDSIG, ZipFile.Source.findEND will sanity check that an ENDSIG indicates a LOCOFF and CENOFF that actually contains LOCSIG and CENSIG.

We currently have testing for the case where the ENDSIG is the start of an actual END header with some padded bytes.

However, we do not have any testing for the case where the ENDSIG found is invalid in that it does not resolve LOCOFF and CENOFF with expected signatures.