RFR: 8365891: failed: Completed task should not be in the queue

[Vladimir Kozlov]

Added missing `task->set_next(nullptr)` for "stale" tasks.

Testing: tier1-3,xcomp,stress

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Commit messages:
 - 8365891: failed: Completed task should not be in the queue

Changes: https://git.openjdk.org/jdk/pull/26872/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365891
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26872/head:pull/26872

PR: https://git.openjdk.org/jdk/pull/26872