Assert failure when retrying failed ADD COLUMN statement

connect 'jdbc:derby:memory:db;create=true';
autocommit off;
create table t(x int);
insert into t values 1, 2;
-- the next statement fails because of overflow (expected)
alter table t add column y smallint generated always as (30000 * x);
-- the next statement fails runs into an assert failure
alter table t add column z int;