row_undo_mod_parse_undo_rec(): Relax the too strict assertion and
correct the comment.

innodb.innodb-blob: Force a flush of the redo log right before
killing the server, to ensure that the code path gets exercised.
(The bogus debug assertion failed on the rollback of the statement
UPDATE t3 SET c=REPEAT('j',3000) WHERE a=2 which did not modify
any indexes before the server was killed.)