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
static size_t max_pipeline_length(struct Curl_multi *multi)
{
  return multi ? multi->max_pipeline_length : 0;
}

static size_t max_pipeline_length(struct Curl_multi *multi)
{
  return multi ? multi->max_pipeline_length : 0;
}
