RFR: 8340401: DcmdMBeanPermissionsTest.java and SystemDumpMapTest.java fail with assert(_stack_base != nullptr) failed: Sanity check [v9]

[Simon Tooke]

> This PR addresses JDK-8340401 by adding a test for a thread with no stack currently assigned.

Simon Tooke has updated the pull request incrementally with one additional commit since the last revision:

  implement and use new Thread::stack_base_or_null() method

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21091/files
  - new: https://git.openjdk.org/jdk/pull/21091/files/8af708a3..4e4f0fe3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21091&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21091&range=07-08

  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21091.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21091/head:pull/21091

PR: https://git.openjdk.org/jdk/pull/21091