Remove several calls to free (#2308)

src/curl_util.cpp

@@ -71,6 +71,7 @@ struct curl_slist* curl_slist_sort_insert(struct curl_slist* list, const char* k
     }
 
     struct curl_slist* new_item;
+    // Must use malloc since curl_slist_free_all calls free.
     if(nullptr == (new_item = static_cast<struct curl_slist*>(malloc(sizeof(*new_item))))){
         free(data);
         return list;