bgpd: Validate both nexthop information (NEXTHOP and NLRI)

bgpd: Validate both nexthop information (NEXTHOP and NLRI)

If we receive an IPv6 prefix e.g.: 2001:db8:100::/64 with nextop: 0.0.0.0, and
mp_nexthop: fc00::2, we should not treat this with an invalid nexthop because
of 0.0.0.0. We MUST check for MP_REACH attribute also and decide later if we
have at least one a valid nexthop.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit a0d2734e879f78fbef5f1815429de331b9940c73)