Add seemingly functional but hacky gc

src/special.c

@@ -22,6 +22,7 @@ struct ucl_object *ucl_special_let(struct ucl_state *state, struct ucl_object *a
 
         if (value->type == UCL_TYPE_ERROR) {
             // TODO cleanup
+            ucl_state_delete(let_state);
             return value;
         }
         ucl_state_put(let_state, sym->symbol, value);