zone-update: fix memory leak in zone_update_clear if full update

Can be reproduced by server shutdown after failed initial zone-commit.

src/knot/updates/zone-update.c

@@ -333,6 +333,7 @@ void zone_update_clear(zone_update_t *update)
 		changeset_clear(&update->change);
 	} else if (update->flags & UPDATE_FULL) {
 		assert(update->new_cont_deep_copy);
+		update_cleanup(update->a_ctx);
 		zone_contents_deep_free(update->new_cont);
 	}
 	free(update->a_ctx);