RFR: 8264605: vmTestbase/nsk/jvmti/SuspendThread/suspendthrd003/TestDescription.java failed with "agent_tools.cpp, 471: (foundThread = (jthread) jni_env->NewGlobalRef(foundThread)) != NULL"

[Leonid Mesnik]

The agentThread variable might have been set by the new agentThread iteration and then deleted in the wrapper from the previous iteration.

The deletion of agentThread variable should be synced with the termination of the agent thread.

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Commit messages:
 - merge
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/6515/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6515&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264605
  Stats: 27 lines in 2 files changed: 5 ins; 15 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6515.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6515/head:pull/6515

PR: https://git.openjdk.java.net/jdk/pull/6515