[jdk11u-dev] RFR: 8322511: JfrCheckpointThreadClosure::do_thread crashes when fetching thread_id

[唐佳未]

Threads which are in the state of `is_attaching_via_jni` may have the NULL object as their `_threadObj` . 
So the JVM will crash when calling `JfrCheckpointThreadClosure::do_thread()`.

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Commit messages:
 - 8322511: JfrCheckpointThreadClosure::do_thread crashes when fetching thread_id

Changes: https://git.openjdk.org/jdk11u-dev/pull/2413/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2413&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322511
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2413.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2413/head:pull/2413

PR: https://git.openjdk.org/jdk11u-dev/pull/2413