rip6_input() inp validation after epoch(9)

After r335924 rip6_input() needs inp validation to avoid
working on FREED inps.

Apply the relevant bits from r335497,r335501 (rip_input() change)
to the IPv6 counterpart.

PR: 232194
Reviewed by: rgrimes, ae (,hps)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17594