Curl_close: clear data->multi_easy on free to avoid use-after-free

Regression from b46cfbc068 (7.59.0)
CVE-2018-16840
Reported-by: Brian Carpenter (Geeknik Labs)

Bug: https://curl.haxx.se/docs/CVE-2018-16840.html
CURLcode Curl_upkeep(struct conncache *conn_cache,
                          void *data)
{
  /* Loop over every connection and make connection alive. */
  Curl_conncache_foreach(data,
                         conn_cache,
                         data,
                         conn_upkeep);
  return CURLE_OK;
}

CURLcode Curl_upkeep(struct conncache *conn_cache,
                          void *data)
{
  /* Loop over every connection and make connection alive. */
  Curl_conncache_foreach(data,
                         conn_cache,
                         data,
                         conn_upkeep);
  return CURLE_OK;
}
