AV when ON CONNECT triggers uses EXECUTE STATEMENT ON EXTERNAL

Test case is based on user sample 
https://groups.google.com/g/firebird-support/c/BoIEtJBkAQQ

1. Create ON CONNECT trigger in any database (except of employee):

```
create trigger trg_connect
  on connect
as
declare id int;
begin
  execute statement 'select 1 from rdb$database'
    on external 'inet://employee'
	into :id;
end
```

2. Connect to this database - Firebird crashed