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
proxy_info_matches(const struct proxy_info* data,
                   const struct proxy_info* needle)
{
  if((data->proxytype == needle->proxytype) &&
     (data->port == needle->port) &&
     Curl_safe_strcasecompare(data->host.name, needle->host.name))
    return TRUE;

  return FALSE;
}

proxy_info_matches(const struct proxy_info* data,
                   const struct proxy_info* needle)
{
  if((data->proxytype == needle->proxytype) &&
     (data->port == needle->port) &&
     Curl_safe_strcasecompare(data->host.name, needle->host.name))
    return TRUE;

  return FALSE;
}
