RFR: 8316895: SeenThread::print_action_queue called on a null pointer

[Daniel Jeliński]

This patch fixes a crash in `test/hotspot/jtreg/runtime/logging/RedefineClasses.java` when the code is compiled without `-fno-delete-null-pointer-checks`.

The test crashes because the method `print_action_queue` is called on a null object reference, which leads to undefined behavior. Some C++ compilers take that as a permission to assume that `this` is never null and remove all such checks.

The patch simply changes `print_action_queue` to a static method taking a pointer to what was formerly `this`. There's not much that could possibly go wrong.

Should I add the bug ID to `test/hotspot/jtreg/runtime/logging/RedefineClasses.java`? The existing code does not crash now; it would only start crashing after [JDK-8316893](https://bugs.openjdk.org/browse/JDK-8316893) is addressed.

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Commit messages:
 - Fix print_action_queue

Changes: https://git.openjdk.org/jdk/pull/15932/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15932&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316895
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15932.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15932/head:pull/15932

PR: https://git.openjdk.org/jdk/pull/15932