CVE-2017-13007/PKTAP: Pass a properly updated struct pcap_pkthdr to the sub-dissector.

The sub-dissector expects that the length and captured length will
reflect the actual remaining data in the packet, not the raw amount
including the PKTAP header; pass an updated header, just as we do for
PPI.

This fixes a buffer over-read discovered by Yannick Formaggio.

Add a test using the capture file supplied by the reporter(s).