Problem is that not all conflicting transactions have THD object.
Therefore, it must be checked that victim has THD
before it's identification is added to victim list as victim's
thread identification is later requested using thd_get_thread_id
function that requires that we have valid pointer to THD object
in trx->mysql_thd.

Victim might not have trx->mysql_thd in two cases:

(1) An incomplete transaction that was recovered from undo logs
on server startup (and not yet rolled back).

(2) Transaction that is in XA PREPARE state and whose client
connection was disconnected.

Neither of these can complete before lock_wait_wsrep()
releases lock_sys.latch.

(1) trx_t::commit_in_memory() is clearing both
trx_t::state and trx_t::is_recovered before it invokes
lock_release(trx_t*) (which would be blocked by the exclusive
lock_sys.latch that we are holding here). Hence, it is not
possible to write a debug assertion to document this scenario.

(2) If is in XA PREPARE state, it would eventually be rolled
back and the lock conflict would be resolved when an XA COMMIT
or XA ROLLBACK statement is executed in some other connection.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>