[jdk8u-dev] RFR: JDK-8183925: Replace ThreadLocalStorage::thread with Thread::current_or_null in jdk8 backport of JDK-8183925

[Ben Taylor]

This change replaces 3 instances of `ThreadLocalStorage::thread` with `Thread::current_or_null` to ensure that `ThreadLocalStorage::is_initialized` is checked before attempting to get the current thread.

Change passes all tier1 tests locally on Linux x86_64 and Windows x86_64

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Commit messages:
 - JDK-8339882: Replace ThreadLocalStorage::thread with Thread::current_or_null in jdk8 backport of JDK-8183925

Changes: https://git.openjdk.org/jdk8u-dev/pull/576/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8183925
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/576.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/576/head:pull/576

PR: https://git.openjdk.org/jdk8u-dev/pull/576