BGP: Fix parsing the AIGP attribute

BGP: Fix parsing the AIGP attribute

There was an invalid ND_TCHECK_LEN(), thus the AIGP metric attribute
at the end of an update message was incorrectly shown as truncated.

Remove it because the data fetching is secured by GET_BE_U_8 and
print_unknown_data.

Add a test file with AIGP attribute at the end of an update message.

This should fix GitHub issue #975.

Remark:
This code need more updates on length checks to change some 'truncated'
process to 'invalid' process.

(cherry picked from commit 255c1cbcf3300216bcd23a0df63493ae37e6d70f)