Problem was assertion assuming we always hold
THD::LOCK_thd_data mutex that is not true.
In most cases this is true but function is
also used from InnoDB lock manager and
there we can't take THD::LOCK_thd_data to
obey mutex ordering. Removed assertion as
wsrep transaction state can't change even
that case.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>