Test MW-286 occasionally failed with error the following message:

```
safe_mutex: Found wrong usage of mutex 'LOCK_wsrep_thd' and 'LOCK_thd_kill'
Mutex currently locked (in reverse order):
LOCK_thd_kill                     mariadb-server/sql/sql_class.h  line 3535
LOCK_wsrep_thd                    mariadb-server/sql/wsrep_thd.cc  line 88
```

The fix consists in calling thd->reset_killed() in wsrep_mysql_parse() after
LOCK_wsrep_thd is unlocked. Which avoids the taking locks LOCK_wsrep_thd and
LOCK_thd_kill in reverse order.