Fix for bug#53541. We need to check whether the slot has been freed or not

before trying to access the transaction instance.

rb://336

storage/innobase/srv/srv0srv.c

@@ -2322,7 +2322,15 @@ srv_lock_check_wait(
 
 			srv_sys_mutex_enter();
 
-			slot_trx = thr_get_trx(slot->thr);
+			/* If the slot has been freed and is not being reused
+			then the slot->thr entry should be NULL. */
+			if (slot->thr != NULL) {
+				ut_a(slot->in_use)
+				slot_trx = thr_get_trx(slot->thr);
+			} else {
+				ut_a(!slot->in_use)
+				slot_trx = NULL;
+			}
 
 			/* We can't compare the pointers here because the
 			memory can be recycled. Transaction ids are not