RFR: 8310530: PipedOutputStream.flush() accesses sink racily

[Sergey Tsypanov]

Just a tiny clean-up to remove racy read within synchronized method

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Commit messages:
 - 8310530: PipedOutputStream.flush() accesses sink racily

Changes: https://git.openjdk.org/jdk/pull/14589/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14589&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310530
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14589.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14589/head:pull/14589

PR: https://git.openjdk.org/jdk/pull/14589