Assertion failure srv_undo_tablespaces > 1

If InnoDB is set up with innodb_undo_tablespaces=2 and later restarted with innodb_undo_tablespaces=0, a debug assertion can fail in trx_assign_rseg_low() when a transaction is about to write undo log.

Instead of letting the assertion fail when innodb_undo_tablespaces=0, we should simply skip rollback segments that are outside the system tablespace.