Problem:
---------
A table using pre mysql-5.0 decimal type triggers a segmentation
fault during in-place upgrade to mysql-8.0. The segmentation fault
is caused during the attempt to access the TABLE object before
opening the table in ' Field_decimal::reset().'.

Analysis:
----------
Pre 5.0 decimal type is not supported from mysql-5.0. Tables
containing such un supported decimal type should be fixed by
recreating the table using dump/restore method.

CHECK TABLE command does not detect and give warnings for pre
mysql-5.0 decimal types.

Fix:
-----
In mysql-5.7, the fix adds warning for pre mysql-5.0 decimal types.
User will get the following message when executing CHECK TABLE command

"Table upgrade required for `db_name`.`table_name`.
Please dump/reload table to  fix it!"

mysql-8.0 will not allow in-place upgrade to 8.0 if pre mysql-5.0
decimal type is found in the data directory.

Note: mysql_upgrade will report the above error for tables with old
decimal. mysql_upgrade will not fix old decimal types in mysql-5.7.
'REPAIR TABLE' OR 'ALTER TABLE ... FORCE' will not fix the old decimal
types in mysql-5.7. Tables containing such un supported decimal type
should be fixed by recreating the table using dump/restore method.