ospf6d: solve segfaults with ospf6d on FreeBSD
Do not send ospf6d hellos on fresh created interfaces without
configuration (ie. no vlan configured). Ospf6d use ip6_mtu, if it's not
initalised, Ospf6d tries to alloc indefinite size of memory.
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>