In rtadvd, interface lookup calls if_indextoname() many times in a loop,
(it takes a long time on systems with many interfaces)
without reason and without checking its return value.

Reviewed by: cem
Obtained from: Maryse Levavasseur <maryse.levavasseur@stormshield.eu>
MFC after: 1 month
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D6979