RFR: 8332154: Memory leak in SynchronousQueue

[Viktor Klang]

Local testing seems to indicate that this fix (which mirrors what's done in the FIFO mode) addresses the problem.

But with that said, I haven't come up with a decent way of adding some form of regression test. Suggestions are most welcome. /cc @DougLea

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Commit messages:
 - Memory leak in unfair mode of SynchronousQueue addressed

Changes: https://git.openjdk.org/jdk/pull/19271/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19271&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332154
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19271.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19271/head:pull/19271

PR: https://git.openjdk.org/jdk/pull/19271