MariaDB 10.2 crashes on corrupted SYS_TABLES.MIX_LEN field

MySQL 5.7 introduced a change that broke compatibility with old data files, by reading the SYS_TABLES.MIX_LEN column also for ROW_FORMAT=REDUNDANT tables, even though this column was written as uninitialized garbage in MySQL 4.1 and earlier versions.

Because MariaDB 10.2 does not support the TABLESPACE attribute for InnoDB tables like MySQL 5.7 does, we do not need this change and can essentially revert it.