Clean premaster_secret for GOST

Ensure OPENSSL_cleanse() is called on the premaster secret value calculated for GOST.

With thanks to the Open Crypto Audit Project for reporting this issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit b7ee4815f2)

Conflicts:
	ssl/s3_srvr.c

ssl/s3_srvr.c

@@ -2914,6 +2914,7 @@ int ssl3_get_client_key_exchange(SSL *s)
                                                         s->
                                                         session->master_key,
                                                         premaster_secret, 32);
+        OPENSSL_cleanse(premaster_secret, sizeof(premaster_secret));
         /* Check if pubkey from client certificate was used */
         if (EVP_PKEY_CTX_ctrl
             (pkey_ctx, -1, -1, EVP_PKEY_CTRL_PEER_KEY, 2, NULL) > 0)