CVE-2017-13035/Properly handle IS-IS IDs shorter than a system ID (MAC address).

Some of them are variable-length, with a field giving the total length,
and therefore they can be shorter than 6 octets.  If one is, don't run
past the end.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s), modified
so the capture file won't be rejected as an invalid capture.