RFR: 8334394: Race condition in Class::protectionDomain

[Weijun Wang]

Make sure `pd` is always the same object when `getProtectionDomain0` is null.

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Commit messages:
 - chmod
 - the fix

Changes: https://git.openjdk.org/jdk/pull/19752/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19752&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334394
  Stats: 84 lines in 2 files changed: 67 ins; 10 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/19752.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19752/head:pull/19752

PR: https://git.openjdk.org/jdk/pull/19752