miss "subquery returns more than 1 row" check when MaxChunkSize=1

[lysu]

This issue will be fixed with #7141

1. What did you do?

create table t(id int primary key, v int);
insert into t values(1, 1), (2, 2), (3, 3);
set @@tidb_max_chunk_size=1;
select (select t.id from t where t.id = t.v and t.v != s.id) from t s;

2. What did you expect to see?

ERROR 1105 (HY000): subquery returns more than 1 row

3. What did you see instead?

+-------------------------------------------------------+
| (select t.id from t where t.id = t.v and t.v != s.id) |
+-------------------------------------------------------+
|                                                     2 |
|                                                     1 |
|                                                     1 |
+-------------------------------------------------------+
3 rows in set (0.01 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: None
Git Commit Hash: None
Git Branch: None
UTC Build Time: None
GoVersion: None
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e

[zz-jason]

@lysu We should fix this issue in an individual PR and cherrypick that PR to release-2.0

[lysu]

@zz-jason ok~

[lysu]

fixed by #7375