[CBRD-24023] fix assert for a migration tool for created or updated catalog tables

[beyondykk9]

http://jira.cubrid.org/browse/CBRD-24023

The catalog record representaion's var-part can consist with 2-byte offset length when the catalog table is altered by SQL statment like as "ALTER TABLE _db_class ADD COLUMN unique_name VARCHAR(255) AFTER class_of". Therefore, we need to fix 'assert' for a migration tool for created or updated catalog tables.