apacheGH-40276: [C++] Fix an simple buffer-overflow case in decimal_b…

…enchmark (apache#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: apache#40276

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

cpp/src/arrow/util/decimal_benchmark.cc

@@ -131,7 +131,7 @@ static void BinaryMathOpAggregate(
 
   for (auto _ : state) {
     BasicDecimal128 result;
-    for (int x = 0; x < 100; x++) {
+    for (int x = 0; x < kValueSize; x++) {
       result += v[x];
     }
     benchmark::DoNotOptimize(result);