CVE-2017-13033/VTP: Add more bound and length checks.

This fixes a buffer over-read discovered by Bhargava Shastry.

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.

Update another VTP test's .out file for this change.

Don't treate a TLV type or length of 0 as invalid; a type of 0 should
just be reported as illegal if that type isn't used, and the length is
the length of the *value*, not the length of the entire TLV, so if it's
zero there won't be an infinite loop.  (It's still not *legal*, as the
values of all the TLVs we handle are 1 16-bit word long; we added a
check for that.)

Update some comments while we're at it, to give a new URL for one Cisco
page and a non-Cisco URL for another former Cisco page (Cisco's UniverCD
pages don't seem to be available any more, and Cisco's robots.txt file
didn't allow the Wayback Machine to archive it).