eng_devcrypto: expand digest failure cases

Return failure when the digest_ctx is null in digest_update and
digest_final, and when md is null in digest_final.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7585)