RFR: JDK-8261873: Do not start a deleted thread in jfrRecorderThread.cpp start_thread

[Matthias Baesken]

In jfrRecorderThread.cpp start_thread there is a code path where potentially
Thread::start is called on a deleted thread, this should be avoided.

See also the related Sonar finding :

https://sonarcloud.io/project/issues?id=shipilev_jdk&languages=cpp&open=AXck7_QbBBG2CXpcnIyR&resolved=false&severities=BLOCKER&types=BUG

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Commit messages:
 - JDK-8261873

Changes: https://git.openjdk.java.net/jdk/pull/2603/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2603&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261873
  Stats: 4 lines in 1 file changed: 3 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2603.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2603/head:pull/2603

PR: https://git.openjdk.java.net/jdk/pull/2603