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GH-45180: [C++][Fuzzing] Fix Negation bug discovered by fuzzing (#45181)



### Rationale for this change
If the value for Decimal32 or Decimal64 is `INT32_MIN` or `INT64_MIN` respectively, then UBSAN reports an issue when calling Negate on them due to overflow. 

### What changes are included in this PR?
Have the `Negate` methods of Decimal32 and Decimal64 use `arrow::internal::SafeSignedNegate`.

### Are these changes tested?
Unit tests were added for both cases which were able to reproduce the problem when UBSAN was on without the fix.

### Are there any user-facing changes?
No.

* OSS-Fuzz issue: https://issues.oss-fuzz.com/issues/371239168

* GitHub Issue: #45180

Authored-by: Matt Topol <zotthewizard@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>