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_setup_connection(struct connectdata *conn)
{
  struct Curl_easy *data = conn->data;
  CURLcode result;

  /* Clear the TLS upgraded flag */
  conn->tls_upgraded = FALSE;

  /* Initialise the SMTP layer */
  result = smtp_init(conn);
  if(result)
    return result;

  data->state.path++;   /* don't include the initial slash */

  return CURLE_OK;
}

static CURLcode smtp_setup_connection(struct connectdata *conn)
{
  struct Curl_easy *data = conn->data;
  CURLcode result;

  /* Clear the TLS upgraded flag */
  conn->tls_upgraded = FALSE;

  /* Initialise the SMTP layer */
  result = smtp_init(conn);
  if(result)
    return result;

  data->state.path++;   /* don't include the initial slash */

  return CURLE_OK;
}
