RFR: 8279949: JavaThread::_free_handle_block leaks native memory

[Leonid Mesnik]

Please review following fix which delete whole list of JNIHandle blocks in JNIHandleBlock::release_block(...).
Also, I added sanity verification of _pop_frame_link to ensure that there are no leaks there.

Fix verified with tier1-6. Also, verified that memory leak is not reproduced anymore.

Thanks to Vladimir I.  for finding exact root cause of problem.

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Commit messages:
 - 8279949: JavaThread::_free_handle_block leaks native memory

Changes: https://git.openjdk.java.net/jdk/pull/7453/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7453&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279949
  Stats: 10 lines in 1 file changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7453.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7453/head:pull/7453

PR: https://git.openjdk.java.net/jdk/pull/7453