(for 4.9.3) CVE-2018-14880/OSPFv3: Fix a bounds check

Need to test bounds check for the last field of the structure lsa6_hdr.
No need to test other fields.

Include Security working under the Mozilla SOS program had independently
identified this vulnerability in 2018 by means of code audit.

Wang Junjie of 360 ESG Codesafe Team had independently identified this
vulnerability in 2018 by means of fuzzing and provided the packet capture
file for the test.