really damaged MyISAM tables

When unpacking a blob column from broken row server crash
could happen. This could rather happen when trying to repair
a table using either REPAIR TABLE or myisamchk, though it
also could happend when trying to access broken row using
other SQL statements like SELECT if table is not marked as
crashed.

Fixed ulong overflow when trying to extract blob from
broken row.

Affects MyISAM only.