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GH-40276: [C++] Fix an simple buffer-overflow case in decimal_benchmark (#40277)



### Rationale for this change
An simple buffer-overflow case found in decimal_benchmark.

### What changes are included in this PR?
Change the loop len from `100` to correct `kValueSize`.

### Are these changes tested?

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

* GitHub Issue: #40276

Authored-by: hugo.zhang <hugo.zhang@openpie.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>