Cause: `table->part_info` is stale after `prep_alter_part_table()` while
`table->m_needs_reopen == true`.
Fix: restore `table->part_info` in case of error in `prep_alter_part_table()`.
Tested with main, parts, innodb suites.
MyISAM only allows online alter if autoincrement didn't change.
MyISAM detects that by comparing new autoinc value from create_info,
with the old one, stored in MYI. But in partitioned tables,
create_info->auto_increment_value is for the whole table, max of
autoinc values of individual MYI partitions. So *some* MYI partitions
will inevitably think that alter table changes auto_increment value
and will deny online alter.
Fix: only compare autoinc values, if the user has used AUTO_INCREMENT
in the ALTER TABLE statement.
