Reported in #1561470 (http://curl.haxx.se/bug/view.cgi?id=1561470), libcurl

would crash if a bad function sequence was used when shutting down after
using the multi interface (i.e using easy_cleanup after multi_cleanup) so
precautions have been added to make sure it doesn't any more - test case 529
was added to verify.

lib/urldata.h

@@ -1053,6 +1053,9 @@ struct UrlState {
      must keep it around and add it to the list of handles to kill once all
      its connections are gone */
   void *shared_conn;
+  bool closed; /* set to TRUE when curl_easy_cleanup() has been called on this
+                  handle, but it is kept around as mentioned for
+                  shared_conn */
 };