fix: 修复update指定表别名时审核错误的问题(fix #249)

[hanchuanchuan]

以下update语句审核有误，会误报列不存在(指定了表别名但未使用)。

# table schema
create table if not exists t1(id int primary key,c1 int);

update t1 a set c1 = 2;

[codecov]

Codecov Report

Merging #250 into master will decrease coverage by 0.0045%.
The diff coverage is 0.0000%.

@@               Coverage Diff                @@
##             master       #250        +/-   ##
================================================
- Coverage   59.9538%   59.9493%   -0.0046%     
================================================
  Files           373        373                
  Lines         83691      83694         +3     
================================================
- Hits          50176      50174         -2     
- Misses        28976      28979         +3     
- Partials       4539       4541         +2