RFR: 8334394: Race condition in Class::protectionDomain [v2]

[Weijun Wang]

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

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  explain why the test is related to the fix

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19752/files
  - new: https://git.openjdk.org/jdk/pull/19752/files/637cf0df..4ce5bc24

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19752&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19752&range=00-01

  Stats: 25 lines in 1 file changed: 25 ins; 0 del; 0 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