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_quit(struct connectdata *conn)
{
  CURLcode result = CURLE_OK;

  /* Send the QUIT command */
  result = Curl_pp_sendf(&conn->proto.smtpc.pp, "%s", "QUIT");

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

  return result;
}

static CURLcode smtp_perform_quit(struct connectdata *conn)
{
  CURLcode result = CURLE_OK;

  /* Send the QUIT command */
  result = Curl_pp_sendf(&conn->proto.smtpc.pp, "%s", "QUIT");

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

  return result;
}
