copr: tikv crash when elt enum type

[wshwsh12]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table t;
create table t(a enum("a"), b int);
insert into t values(1,1);
select * from t where elt(a,b);

2. What did you expect to see? (Required)

MySQL [test]> select * from t where elt(a,b);
+------+------+
| a | b |
+------+------+
| a | 1 |
+------+------+
1 row in set (0.001 sec)

3. What did you see instead (Required)

ERROR 9002 (HY000): TiKV server timeout

4. What is your TiDB version? (Required)

Only master

[ti-srebot]

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