CTL: Add length validation for incoming HA messages.

This should fix uninitialized memory reads when working with broken
HA peer, like one fixed in 1a8d8a3a909. Instead print error message
and kill the HA link.

MFC after: 3 days
Sponsored by: iXsystems, Inc.

(cherry picked from commit 530d274c15e5b3f69088b4b53f8dc5b2b849a916)