RFR: 8360201: JFR: Initialize JfrThreadLocal::_sampling_critical_section

[Aleksey Shipilev]

Initially found by UBSan. I was not able to reproduce it locally, @MBaesken would try. But the problem seems obvious: the initial value for the `bool` field is garbage. I have checked other fields in `JfrThreadLocal`, they seem fine.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `jdk_jfr`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25938/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25938&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360201
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25938.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25938/head:pull/25938

PR: https://git.openjdk.org/jdk/pull/25938