zebra: fix FreeBSD warning on fresh OS boot

zebra: fix FreeBSD warning on fresh OS boot

Handle corner case where a warning log message is issued on interface
address netmask handling with sockaddr type AF_LINK: it may come empty
or with match all (all 0xFF).

In the first case all lengths are zero and we only need to copy the
first bytes, second case it comes with a zero index and all 0xFF bytes.

In any case we only need to figure out a few of the first bytes instead
of all data.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>