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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31302