Add seemingly functional but hacky gc

src/utility.c

@@ -77,6 +77,7 @@ struct ucl_object *ucl_list_nth(struct ucl_object *list, int n) {
 
     int length = ucl_list_length(list)->integer;
     UCL_COND_OR_RET_ERROR(length > n, "Position n >= list length in ucl_list_nth");
+    UCL_COND_OR_RET_ERROR(n >= 0, "Index to ucl_list_nth was less that zero");
 
     struct ucl_object *node = list;
     for (int i = 0; i < n; i++) {