fix function substr with 3 arguments bug (#1296)

[ti-srebot]

cherry-pick #1296 to release-3.1

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What problem does this PR solve?

Issue Number: close #xxx

Problem Summary: tiflash funcion substr with 3 arguments has bug

create table t (c varchar(64));
ALTER TABLE t SET TIFLASH REPLICA 1;
insert into t values ("ABC"), ("DEF"), ("");
insert into t select * from t;
insert into t select * from t;
insert into t select * from t;
insert into t select * from t;
insert into t select * from t;
insert into t select * from t;

analyze table t;
select substr(c, 2), count(1) from t group by substr(c, 2);

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

[marsishandsome]

/run-all-tests