ppp: Reject invalid endpoint discriminator options

Per RFC1717 section 5.1.3, the option length must be at least three.
Processing an undersized option would trigger a large out-of-bounds
write.

PR: 271910
Reported by: Robert Morris
Reported by: Décio Brandão (0xDBJ)
Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58554

(cherry picked from commit b9d07a4308226b683b64827e0aaed1180e0da996)