Fix nd6 rib_action() handling.

rib_action() guarantees valid rc filling IFF it returns without error.
Check rib_action() return code instead of checking rc fields.

PR: 253800
Reported by: Frederic Denis <freebsdml@hecian.net>
Approved by: re(gjb)

(cherry picked from commit ea10694336b9a07d58d22187052291976f4906b2)