RFR: 8278519: serviceability/jvmti/FieldAccessWatch/FieldAccessWatch.java failed "assert(handle != __null) failed: JNI handle should not be null"

[Alex Menkov]

The fix adds a check in the event handler which filters out event from non-test threads

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Commit messages:
 - Filter out events from other threads

Changes: https://git.openjdk.java.net/jdk/pull/6874/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6874&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278519
  Stats: 14 lines in 2 files changed: 8 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6874.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6874/head:pull/6874

PR: https://git.openjdk.java.net/jdk/pull/6874