Fix a memory leak in tls1_mac

Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5650)

ssl/record/ssl3_record.c

@@ -1297,8 +1297,10 @@ int tls1_mac(SSL *ssl, SSL3_RECORD *rec, unsigned char *md, int sending)
         mac_ctx = hash;
     } else {
         hmac = EVP_MD_CTX_new();
-        if (hmac == NULL || !EVP_MD_CTX_copy(hmac, hash))
+        if (hmac == NULL || !EVP_MD_CTX_copy(hmac, hash)) {
+            EVP_MD_CTX_free(hmac);
             return 0;
+        }
         mac_ctx = hmac;
     }