Remove a memset
Remove a memset

Also avoids calling EVP_MD_size() and a missing negative result check.

Issue found by Coverity.

Reviewed-by: Andy Polyakov <015edf36635e030d5e009e08808a59984e629740@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6592)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl