nanobench: fix performance counter buffer initialization #35607

Problem: The Linux perf counter buffers are documented as starting with three read_format metadata slots, but std::vector<uint64_t>{3} creates a one-element vector containing 3. In normal benchmark runs this is usually hidden because successful monitor() calls resize the buffers before updateResults(), while failed setup sets mHasError before the indexed reads.

Fix: Use the vector size constructor so the buffers start with three zero-initialized metadata slots. This should not change the usual successful benchmark path, but it makes the default object state match the code's indexing assumptions.

Reproducer: https://godbolt.org/z/scE8rMd8Y Upstream: https://github.com/martinus/nanobench/pull/138

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35607.

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Code review ACK b6b1d0653af18dfff752c03381c9f58e11b83130

This is trivial, and also kind of pointless, because the buffer are always resized appropriately anyway, but the code change is fine.

ACK b6b1d0653af18dfff752c03381c9f58e11b83130