rip: Add missing minimum length validation in rip_output()

If the socket is configured such that the sender is expected to supply
the IP header, then we need to verify that it actually did so.

Reported by: syzkaller+KMSAN
Reviewed by: donner
Sponsored by: The FreeBSD Foundation

(cherry picked from commit ba21825202737a8b7e90e1ef669c7fe7e7d50325)