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

[Simon Tooke]

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

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Commit messages:
 - JDK-8340401: also fix spelling in comment
 - test for thread with no stack assigned
 - revert legend fix (will be in a different PR)
 - update legend text for 'offset'

Changes: https://git.openjdk.org/jdk/pull/21091/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21091&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340401
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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