RFR: 8359870: JVM crashes in AccessInternal::PostRuntimeDispatch

[Kevin Walls]

ThreadDumper/ThreadSnapshot need to handle a failure to resolve the native VM JavaThread from a java.lang.Thread.  This is hard to reproduce but a thread that has since terminated can provoke a crash.  Recognise this and return a null ThreadSnapshot.

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Commit messages:
 - ThreadSnapshot comment
 - Merge remote-tracking branch 'upstream/master' into 8359870_threadexited
 - 8359870: JVM crashes in AccessInternal::PostRuntimeDispatch

Changes: https://git.openjdk.org/jdk/pull/25958/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25958&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359870
  Stats: 10 lines in 4 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25958.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25958/head:pull/25958

PR: https://git.openjdk.org/jdk/pull/25958