(for 4.9.3) CVE-2018-14470/Babel: fix an existing length check

In babel_print_v2() the non-verbose branch for an Update TLV compared
the TLV Length against 1 instead of 10 (probably a typo), put it right.

This fixes a buffer over-read discovered by Henri Salo from Nixu
Corporation.

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