[ecs] Initialize registry href_comp to nullent

This indicates that no hierarchy reference component has been
registered, which must be done by the user when hierarchies are used.

libs/ecs/component.c

@@ -38,6 +38,7 @@ ECS_NewRegistry (void)
 {
 	ecs_registry_t *reg = calloc (1, sizeof (ecs_registry_t));
 	reg->next = Ent_Index (nullent);
+	reg->href_comp = nullent;
 	return reg;
 }