dml: load data can succeed with with wrong utf8 character

[AilinKid]

Bug Report

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1. Minimal reproduce step (Required)

Amended case:
insert into t value( CAST(BINARY(X'c2c2c2c2') AS CHAR(2)));
ERROR 1406 (22001): Data too long for column 'a' at row 1

create table t(a varchar(2)/char(2))
load data local file 'xxxx' into table t     // file contains Binary(X'c2c2c2c2')

When we load data to a column of CHAR(2) of a existed table with data BINARY(X'c2c2c2c2'). It will succeed in
insertion ignoring the data-too-long error. (make it as warning)

Reason: Cast function will throw data-too-long error and return the truncated string equal to X'c2c2', actually the
returned truncated value is still invalid cause it contains the non-utf8 character.

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

Load data should return a error.

3. What did you see instead (Required)

Load data successfully

4. What is your TiDB version? (Required)

master

[ti-srebot]

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