handle unsigned value wrongly in builtinCastIntAsDecimalSig

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

CREATE TABLE t1 (v bigint(20) UNSIGNED NOT NULL);
INSERT INTO t1 VALUES (1), (2);
SELECT SUM(IF(v > 1, v, -v)) FROM t1;

2. What did you expect to see?

1

3. What did you see instead?

+-----------------------+
| SUM(IF(v > 1, v, -v)) |
+-----------------------+
|  18446744073709551617 |
+-----------------------+
1 row in set (0.02 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

+---------------------------------------------+
| version()                                   |
+---------------------------------------------+
| 5.7.25-TiDB-v4.0.0-alpha-5-g1c43f55f9-dirty |
+---------------------------------------------+
1 row in set (0.00 sec)