Remove a memset

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

Issue found by Coverity.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6592)

crypto/sm2/sm2_za.c

@@ -59,8 +59,6 @@ int sm2_compute_userid_digest(uint8_t *out,
         goto done;
     }
 
-    memset(out, 0, EVP_MD_size(digest));
-
     if (!EVP_DigestInit(hash, digest)) {
         SM2err(SM2_F_SM2_COMPUTE_USERID_DIGEST, ERR_R_EVP_LIB);
         goto done;