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
int Curl_removeHandleFromPipeline(struct Curl_easy *handle,
                                  struct curl_llist *pipeline)
{
  if(pipeline) {
    struct curl_llist_element *curr;

    curr = pipeline->head;
    while(curr) {
      if(curr->ptr == handle) {
        Curl_llist_remove(pipeline, curr, NULL);
        return 1; /* we removed a handle */
      }
      curr = curr->next;
    }
  }

  return 0;
}

int Curl_removeHandleFromPipeline(struct Curl_easy *handle,
                                  struct curl_llist *pipeline)
{
  if(pipeline) {
    struct curl_llist_element *curr;

    curr = pipeline->head;
    while(curr) {
      if(curr->ptr == handle) {
        Curl_llist_remove(pipeline, curr, NULL);
        return 1; /* we removed a handle */
      }
      curr = curr->next;
    }
  }

  return 0;
}
