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GH-43211: [C++] Fix decimal benchmarks to avoid out-of-bounds accesses (#43212)

### Rationale for this change

Some of the decimal benchmarks access their benchmark data in strides, without checking that the accesses fall within bounds.

A side effect is that this will break benchmark history because the iterations/s calculation was wrong, even though actual performance is unchanged.

### Are these changes tested?

By the continuous benchmarking jobs.

### Are there any user-facing changes?

No.
* GitHub Issue: #43211

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>