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

  while(smtpc->state != SMTP_STOP && !result)
    result = Curl_pp_statemach(&smtpc->pp, TRUE);

  return result;
}

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

  while(smtpc->state != SMTP_STOP && !result)
    result = Curl_pp_statemach(&smtpc->pp, TRUE);

  return result;
}
