fix cast to decimal wrong sign

[guo-shaoge]

What problem does this PR solve?

Issue Number: close #9301
ref pingcap/tidb#53892

Problem Summary:

cast -0.741(decimal(11, 4)) to decimal(10, 0)

intput Decimal32
    value: -7410
    scale: 4

cast calc process: 
1. calc new value: -7410 / scale_mul(10000) = 0
2. 7410 % 10000 > 5000, so need to round
3. old value is neg, so result should be 0-1 = -1.
    (bug here, should check old value's sign instead new value's sign, because new value canbe zero)

What is changed and how it works?

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[yibin87]

LGTM

[gengliqi]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gengliqi, yibin87

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-08 06:13:40.505025193 +0000 UTC m=+505350.372124274: ☑️ agreed by yibin87.
• 2024-08-09 04:52:26.295526704 +0000 UTC m=+586876.162625787: ☑️ agreed by gengliqi.

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #9356.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #9357.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #9358.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #9359.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #9360.