CastIntAsString did not work in some cases

[xhebox]

Bug Report

Please answer these questions before submitting your issue. Thanks!

cast(int(1) as char) will set char.flen = 1, which may truncate integers. We set char.flen = 11 as mysql if we are casting integers.

1. Minimal reproduce step (Required)

CREATE TABLE `t` (
  `id` int(11) DEFAULT NULL,
   `var` int(1) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
insert into t values(1,-1);
select id, var, cast(var as char) from t;

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

id | var | cast(var as char)
-- | -- | --
1 | -1 | -1

3. What did you see instead (Required)

id | var | cast(var as char)
-- | -- | --
1 | -1 | -

4. What is your TiDB version? (Required)

v6.5.2