In MySQL 5.7, rollback segments 1 to 32 are used for temporary tables,
which is an unnecessary file format change from MySQL 5.6.
This format change was avoided in MariaDB Server by
commit 124bae08 (MDEV-12289).

An upgrade from MySQL 5.7 would crash due to dereferencing a null pointer,
which is a regression due to
commit 0b47c126 (MDEV-13542).

trx_rseg_t::get(): Return nullptr if no tablespace exists. This is where
the upgrade would crash.

trx_rseg_mem_restore(): Return DB_TABLESPACE_NOT_FOUND if the
undo tablespace does not exist. This is likely dead code.