issue of add foreign key failed but doesn't rollback

[crazycs520]

Reproduce

 set @@global.tidb_enable_foreign_key=1;
set @@foreign_key_checks=1;
create table t1 (id int key,a int, index(a));
create table t2 (id int key,a int);
insert into t1 values (1,1);

test> ALTER TABLE t1 ADD foreign key fk(a) references t2(id) ON DELETE CASCADE;
(1452, 'Cannot add or update a child row: a foreign key constraint fails (`test`.`t1`, CONSTRAINT `fk` FOREIGN KEY (`a`) REFERENCES `t2` (`id`) ON DELETE CASCADE)')
test> show create table t1\G
***************************[ 1. row ]***************************
Table        | t1
Create Table | CREATE TABLE `t1` (
  `id` int(11) NOT NULL,
  `a` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */,
  KEY `a` (`a`),
  CONSTRAINT `fk` FOREIGN KEY (`a`) REFERENCES `test`.`t2` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

Add foreign key failed, but show create table result still contain the foreign key info.

TiDB Version

***************************[ 1. row ]***************************
tidb_version() | Release Version: v6.5.0-alpha-248-g6b4739f170
Edition: Community
Git Commit Hash: 6b4739f170a1fb53a10d3f4bd50ad6288460f252
Git Branch: master
UTC Build Time: 2022-12-02 06:31:34
GoVersion: go1.19.3
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: unistore