sometimes it will success when adding a column with invalid default value

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
set sql_mode='';
create table t1(a int);
alter table t1 add column (b int unsigned default '-1');
```

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

Same with MySQL, an error should be reported:

```
mysql> alter table t1 add column (b int unsigned default '-1');
ERROR 1067 (42000): Invalid default value for 'b'
```

### 3. What did you see instead (Required)

Tidb success when adding a column with default value '-1'

### 4. What is your TiDB version? (Required)

master, I think it affects all TiDB versions