bgpd: Do not discard an UPDATE if the global nexthop is set to ::

When we receive an UPDATE with MP_NEXTHOP len as 32 bytes, we shouldn't
check if the global (1st) nexthop is unspecified.

Peering between bird and FRRouting we receive from Bird something like:
```
rcvd UPDATE w/ attr: , origin i, mp_nexthop ::(fe80::a00:27ff:fe09:f8a3)
```
The link-local (2nd) nexthop is valid and validated later in the code.

Before it was marked:
```
IPv6 unicast -- DENIED due to: martian or self next-hop;
```

After it's a valid prefix:
```
spine1-debian-9# show bgp
BGP table version is 0, local router ID is 2.2.2.2, vrf id 0
Default local pref 100, local AS 65002
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
   2a02:4780::/64   fe80::a00:27ff:fe09:f8a3
                                                           0 65001 i

Displayed  1 routes and 1 total paths
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

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