hash table UPDATE optional cb for HT free

Allows freeing items in the hash table.
diff --git a/src/tree_data_free.c b/src/tree_data_free.c
index bf17a91..806ad3c 100644
--- a/src/tree_data_free.c
+++ b/src/tree_data_free.c
@@ -166,7 +166,7 @@
         ly_free_prefix_data(opaq->format, opaq->val_prefix_data);
     } else if (node->schema->nodetype & LYD_NODE_INNER) {
         /* remove children hash table in case of inner data node */
-        lyht_free(((struct lyd_node_inner *)node)->children_ht);
+        lyht_free(((struct lyd_node_inner *)node)->children_ht, NULL);
         ((struct lyd_node_inner *)node)->children_ht = NULL;
 
         /* free the children */