and a char-field > 128 exists

CHECK TABLE (non-QUICK) and any form of repair table did wrongly rate
records as corrupted under the following conditions:
1. The table has dynamic row format and
2. it has a CHAR like column > 127 bytes (but not VARCHAR)
   (for multi-byte character sets this could be less than 127
   characters) and
3. it has records with > 127 bytes significant length in that column
   (a byte beyond byte position 127 must be non-space).
Affected were the statements CHECK TABLE, REPAIR TABLE, OPTIMIZE TABLE,
ALTER TABLE. CHECK TABLE reported and marked the table as crashed if any
record was present that fulfilled condition 3. The other statements
deleted these records.

The problem was a signed/unsigned compare in MyISAM code. A
char to uchar change became necessary after the big byte to uchar
change.