data tree BUGFIX possible illegal pointer cleared

Fixes #390

Author: michalvasko

src/tree_data.c

@@ -4303,6 +4303,9 @@ lyd_validate(struct lyd_node **node, int options, void *var_arg)
         if (to_free) {
             if ((*node) == to_free) {
                 *node = NULL;
+                if (data_tree == to_free) {
+                    data_tree = NULL;
+                }
             }
             lyd_free(to_free);
             to_free = NULL;