ktls: Use ATF_REQUIRE instead of assert() for validating TLS header lengths.

The TLS header length field is set by the kernel, so if it is
incorrect that is an indication of a kernel bug, not an internal error
in the tests.

Prompted by: markj (comment in an earlier review)
Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D33003