Using `innodb_thread_concurrency` will call `wsrep_thd_is_aborting` to
check WSREP thread state. This call should be protected by taking
`LOCK_thd_data` before entering function.
Applier and TOI threads should no be affected with usage of
`innodb_thread_concurrency` variable so returning before any checks.
Reviewed-by: Jan Lindström <jan.lindstrom@mariadb.com>