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GH-42140: [C++] Avoid invalid accesses in parquet-encoding-benchmark (#42141)

### Rationale for this change

`parquet-encoding-benchmark`  would make invalid memory accesses if more than one repetition per benchmark is requested by the user.

This was initially noticed in https://github.com/apache/arrow/pull/41205 : switching to jemalloc would crash the specific benchmark(s) on a macOS ARM machine.

### What changes are included in this PR?

Make sure benchmark data initialization is idempotent.

### Are these changes tested?

Locally using Valgrind.

### Are there any user-facing changes?

No.
* GitHub Issue: #42140

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