mysql/mysql-server@e00ad49edc8b07317b52c9efd0810f2cbc57877a
which introduced WL#6326 to MySQL 5.7.2 added a buffer page
acquisition to CHECK TABLE code (solely for the purpose of
obeying the changed latching order), but failed to check that
a parent page actually exists. It would not necessarily exist in a
corrupted index where a parent page is missing pointer records
to child pages.