RFR: 8273381: Assert in PtrQueueBufferAllocatorTest.stress_free_list_allocator_vm

[Kim Barrett]

Please review this fix of the test, eliminating a race that could
unexpectedly leave buffers in the "completed" list after all the threads
have shut down.

Testing:
mach5 tier1

-------------

Commit messages:
 - fix race in test

Changes: https://git.openjdk.java.net/jdk/pull/5820/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5820&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273381
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5820.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5820/head:pull/5820

PR: https://git.openjdk.java.net/jdk/pull/5820