If an operation would generate a MAC output (e.g. for digest operation
or for an AEAD or EtA operation), then an empty payload buffer is
valid. Only reject requests with an empty buffer for "plain" cipher
Some of the AES-CCM NIST KAT vectors use an empty payload.
While here, don't advance crp_payload_start for requests that use an
empty payload with an inline IV. (*)
Reported by: firstname.lastname@example.org (*)
Sponsored by: The FreeBSD Foundation