Fixed by moving update_key_parts() down to be after write_index().


myisam/sort.c:
  write_index() collects index statistic which is further used in
  update_key_parts(). Thus update_key_parts() must be called after
  write_index().
mysql-test/r/repair.result:
  Test case for bug#18874.
mysql-test/t/repair.test:
  Test case for bug#18874.