RFR: 7903504: JMH: Fix new Sonar warnings

[Aleksey Shipilev]

SonarCloud reports a few warnings in new code.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jmh/pull/113/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=113&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903504
  Stats: 15 lines in 4 files changed: 2 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jmh/pull/113.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/113/head:pull/113

PR: https://git.openjdk.org/jmh/pull/113