MDEV-24967 : Signal 11 on ha_innodb.cc::bg_wsrep_kill_trx line 18611

Null poiter reference in case where bf_thd has no trx .e.g. when
we have MDL-conflict.

storage/innobase/handler/ha_innodb.cc

@@ -18688,7 +18688,9 @@ static void bg_wsrep_kill_trx(void *void_arg)
 		/* apparently victim trx was meanwhile rolled back.
 		tell bf thd not to wait, in case it already started to */
 		trx_t *trx= thd_to_trx(bf_thd);
-		if (lock_t *lock= trx->lock.wait_lock) {
+		/* note that bf_thd might not have trx e.g. in case of
+		MDL-conflict. */
+		if (lock_t *lock= (trx ? trx->lock.wait_lock : NULL)) {
 			trx_mutex_enter(trx);
 			lock_cancel_waiting_and_release(lock);
 			trx_mutex_exit(trx);