frm and data files for tables created by earlier MySQL
versions becomes out of sync after certain ALTER TABLE statements:
- One that changes column default value;
- One that changes table comment;
- One that changes table password.

As a result one can expirience either server crash or data corruption/loss.

This fix ensures that running ALTER TABLE on tables created by earlier
MySQL versions recreates data files.


sql/sql_table.cc:
  Ensure that running ALTER TABLE on tables created by earlier
  MySQL versions recreates data files.