ospf6d: fix regression detected by topotest

ospf6d: fix regression detected by topotest

With the old API, ospf6d always needed to send a nexthop address and a
nexthop interface when advertising a route to zebra. In the case where
the nexthop address didn't exist (e.g. connected route), zebra would
take care of ignore it in the zread_ipv6_add() function.

Now, if we have a nexthop interface but not a nexthop address, we not
only can but we should send a nexthop of type NEXTHOP_TYPE_IFINDEX. zebra
won't fix bad nexthops anymore because the clients have a proper API to
send correct messages.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>