RFR: 8364316: JFR: Incorrect validation of mirror fields

[Erik Gahlin]

Could I please have a review of a PR that fixes a bug in the mirror field validation?

Testing: jdk/jdk/jfr

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.org/jdk/pull/26539/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26539&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364316
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/26539.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26539/head:pull/26539

PR: https://git.openjdk.org/jdk/pull/26539