mime: properly unbind mime structure in curl_mime_free().

This allows freeing a mime structure bound to the easy handle before
curl_easy_cleanup().

Fixes #1970.

lib/mime.c

@@ -1130,6 +1130,7 @@ void curl_mime_free(curl_mime *mime)
   curl_mimepart *part;
 
   if(mime) {
+    mime_subparts_unbind(mime);  /* Be sure it's not referenced anymore. */
     while(mime->firstpart) {
       part = mime->firstpart;
       mime->firstpart = part->nextpart;