Assert in PtrQueueBufferAllocatorTest.stress_free_list_allocator_vm

Happened during gtests on x86 in GHAs for an unrelated patch in our SapMachine 18 CI. Delta between SapMachine and stock OpenJDK not different enough to cause this, so I believe this to be an upstream problem.

```
2021-09-05T07:14:24.4105685Z [----------] 2 tests from PtrQueueBufferAllocatorTest
2021-09-05T07:14:24.4106571Z [ RUN ] PtrQueueBufferAllocatorTest.test_vm
2021-09-05T07:14:24.4107504Z [ OK ] PtrQueueBufferAllocatorTest.test_vm (0 ms)
2021-09-05T07:14:24.4108542Z [ RUN ] PtrQueueBufferAllocatorTest.stress_free_list_allocator_vm
2021-09-05T07:14:24.4109347Z Stressing allocator for 1000 ms
2021-09-05T07:14:24.4109773Z allocations: 399676
2021-09-05T07:14:24.4110563Z allocations: 397190
2021-09-05T07:14:24.4111158Z allocations: 410792
2021-09-05T07:14:24.4111562Z allocations: 413994
2021-09-05T07:14:24.4111969Z total allocations: 1621652
2021-09-05T07:14:24.4112418Z allocator free count: 172177
2021-09-05T07:14:24.4113133Z # To suppress the following error report, specify this argument
2021-09-05T07:14:24.4114288Z # after -XX: or in .hotspotrc: SuppressErrorAt=/test_ptrQueueBufferAllocator.cpp:115
2021-09-05T07:14:24.4114996Z #
2021-09-05T07:14:24.4115593Z # A fatal error has been detected by the Java Runtime Environment:
2021-09-05T07:14:24.4116088Z #
2021-09-05T07:14:24.4116944Z # Internal Error (/home/runner/work/SapMachine/SapMachine/jdk/test/hotspot/gtest/gc/shared/test_ptrQueueBufferAllocator.cpp:115), pid=2002, tid=2002
2021-09-05T07:14:24.4118035Z # assert(_completed_list.empty()) failed: completed list not empty
2021-09-05T07:14:24.4118554Z #
2021-09-05T07:14:24.4119938Z # JRE version: OpenJDK Runtime Environment (18.0) (fastdebug build 18-internal+0-tstuefe-8fc8374bdbcd511e057c8c50fe73c9ad39be9ebf)
2021-09-05T07:14:24.4122244Z # Java VM: OpenJDK Server VM (fastdebug 18-internal+0-tstuefe-8fc8374bdbcd511e057c8c50fe73c9ad39be9ebf, mixed mode, tiered, g1 gc, linux-x86)
2021-09-05T07:14:24.4123538Z # Problematic frame:
2021-09-05T07:14:24.4124431Z # V [libjvm.so+0x50d403] PtrQueueBufferAllocatorTest_stress_free_list_allocator_vm_Test::TestBody()+0x1443
2021-09-05T07:14:24.4125182Z #
2021-09-05T07:14:24.4126700Z # Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport %p %s %c %d %P %E" (or dumping to /home/runner/work/SapMachine/SapMachine/build/run-test-prebuilt/test-support/jtreg_test_hotspot_jtreg_tier1_common/scratch/core.2002)
2021-09-05T07:14:24.4127878Z #
2021-09-05T07:14:24.4128357Z # An error report file with more information is saved as:
2021-09-05T07:14:24.4129557Z # /home/runner/work/SapMachine/SapMachine/build/run-test-prebuilt/test-support/jtreg_test_hotspot_jtreg_tier1_common/scratch/hs_err_pid2002.log
2021-09-05T07:14:24.4130370Z #
2021-09-05T07:14:24.4130837Z # If you would like to submit a bug report, please visit:
2021-09-05T07:14:24.4131586Z # https://bugreport.java.com/bugreport/crash.jsp
2021-09-05T07:14:24.4132375Z # The crash happened outside the Java Virtual Machine in native code.
2021-09-05T07:14:24.4133068Z # See problematic frame for where to report the bug.
2021-09-05T07:14:24.4133526Z #
```

Feel free to access the original GHA results:

https://github.com/SAP/SapMachine/pull/942/checks?check_run_id=3516709429