MDEV-14817 Server crashes in prep_alter_part_table() after table lock and multiple add partition [fixes #440]

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.