MDEV-14996 kill during FLUSH TABLES FOR EXPORT causes assert

mysql-test/suite/maria/kill.result

+#
+# MDEV-14996
+#  Assertion `!thd->get_stmt_da()->is_sent() ||
+# thd->killed == KILL_CONNECTION' failed in ha_maria::external_lock
+#
+CREATE TABLE t1 (a INT) ENGINE=Aria;
+connect  con1,localhost,root,,;
+FLUSH TABLE t1 FOR EXPORT;
+KILL CONNECTION_ID();
+ERROR 70100: Connection was killed
+disconnect con1;
+connection default;
+DROP TABLE t1;

mysql-test/suite/maria/kill.test

+--echo #
+--echo # MDEV-14996
+--echo #  Assertion `!thd->get_stmt_da()->is_sent() ||
+--echo # thd->killed == KILL_CONNECTION' failed in ha_maria::external_lock
+--echo #
+
+CREATE TABLE t1 (a INT) ENGINE=Aria;
+--connect (con1,localhost,root,,)
+FLUSH TABLE t1 FOR EXPORT;
+--error ER_CONNECTION_KILLED
+KILL CONNECTION_ID();
+--disconnect con1
+--connection default
+DROP TABLE t1;

storage/maria/ha_maria.cc

@@ -2782,7 +2782,7 @@ int ha_maria::external_lock(THD *thd, int lock_type)
             changes to commit (rollback shouldn't be tested).
           */
           DBUG_ASSERT(!thd->get_stmt_da()->is_sent() ||
-                      thd->killed == KILL_CONNECTION);
+                      thd->killed);
           /* autocommit ? rollback a transaction */
 #ifdef MARIA_CANNOT_ROLLBACK
           if (ma_commit(trn))