Assertion .thd->in_active_multi_stmt in TOI

See stress test attached.

The essential ingredients are just three: the ALTER TABLE mysql.gtid_slave_pos ENGINE=InnoDB at the top (drop it and all 5 runs pass), wsrep_gtid_mode=1 with node_1 as an async replica so the DDL arrives with domain 3, and the 150-iteration DDL loop. slave-parallel-threads/wsrep-slave-threads are carried over from my other tests and I did not verify they're required.