crypto: Fix assertions for digest-only sessions with separate output.

Digest-only sessions do not generate modified payload as an output, so
don't bother asserting anything about the payload with respect to the
output buffer other than the payload output start being zero.

In addition, a verify request on a digest-only session doesn't
generate any output at all so should never have a separate output
buffer.

PR: 252316
Reviewed by: markj
Co-authored-by: Jeremy Faulkner <gldisater@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35578

(cherry picked from commit c71f2370c5d480cf70f12ee276e044681c57aefc)