smtp: use the upload buffer size for scratch buffer malloc

... not the read buffer size, as that can be set smaller and thus cause
a buffer overflow! CVE-2018-0500

Reported-by: Peter Wu
Bug: https://curl.haxx.se/docs/adv_2018-70a2.html
static CURLcode smtp_perform_ehlo(struct connectdata *conn)
{
  CURLcode result = CURLE_OK;
  struct smtp_conn *smtpc = &conn->proto.smtpc;

  smtpc->sasl.authmechs = SASL_AUTH_NONE; /* No known auth. mechanism yet */
  smtpc->sasl.authused = SASL_AUTH_NONE;  /* Clear the authentication mechanism
                                             used for esmtp connections */
  smtpc->tls_supported = FALSE;           /* Clear the TLS capability */
  smtpc->auth_supported = FALSE;          /* Clear the AUTH capability */

  /* Send the EHLO command */
  result = Curl_pp_sendf(&smtpc->pp, "EHLO %s", smtpc->domain);

  if(!result)
    state(conn, SMTP_EHLO);

  return result;
}

static CURLcode smtp_perform_ehlo(struct connectdata *conn)
{
  CURLcode result = CURLE_OK;
  struct smtp_conn *smtpc = &conn->proto.smtpc;

  smtpc->sasl.authmechs = SASL_AUTH_NONE; /* No known auth. mechanism yet */
  smtpc->sasl.authused = SASL_AUTH_NONE;  /* Clear the authentication mechanism
                                             used for esmtp connections */
  smtpc->tls_supported = FALSE;           /* Clear the TLS capability */
  smtpc->auth_supported = FALSE;          /* Clear the AUTH capability */

  /* Send the EHLO command */
  result = Curl_pp_sendf(&smtpc->pp, "EHLO %s", smtpc->domain);

  if(!result)
    state(conn, SMTP_EHLO);

  return result;
}
