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>

(cherry picked from commit 9c4a8d24f0ffd5243fa5c6fe27178f669f16d1f5)