Tidb: ddl: add M>=D checking for decimal column definition with default value

[pcqz]

Bug Report

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1. Minimal reproduce step (Required)

create table t( col decimal(1,2) not null default 0);

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

ERROR 1427 (42000): For float(M,D), double(M,D) or decimal(M,D), M must be >= D (column 'col').

3. What did you see instead (Required)

ERROR 1105 (HY000): runtime error: invalid memory address or nil pointer dereference

4. What is your TiDB version? (Required)

v4.0.8

[TszKitLo40]

I will try to fix it.

[ti-srebot]

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