This assertion failure has been observed in TestThreadFailure when rebuilding the freeset:
```
V [libjvm.so+0x14a1a67] ShenandoahFreeSet::rebuild(unsigned long, unsigned long)+0x277 (shenandoahFreeSet.cpp:1197)
V [libjvm.so+0x14cb4e5] ShenandoahHeap::rebuild_free_set(bool)+0x283 (shenandoahHeap.cpp:3136)
V [libjvm.so+0x148961f] ShenandoahConcurrentGC::op_final_updaterefs()+0x20b (shenandoahConcurrentGC.cpp:1267)
V [libjvm.so+0x1486be6] ShenandoahConcurrentGC::entry_final_updaterefs()+0xb2 (shenandoahConcurrentGC.cpp:376)
V [libjvm.so+0x156f7ef] VM_ShenandoahFinalUpdateRefs::doit()+0x3f (shenandoahVMOperations.cpp:77)
V [libjvm.so+0x177d2af] VM_Operation::evaluate()+0xe1 (vmOperations.cpp:71)
V [libjvm.so+0x17eba92] VMThread::evaluate_operation(VM_Operation*)+0x66 (vmThread.cpp:281)
V [libjvm.so+0x17ec3ca] VMThread::inner_execute(VM_Operation*)+0x3da (vmThread.cpp:435)
V [libjvm.so+0x17ec7fd] VMThread::loop()+0x151 (vmThread.cpp:502)
V [libjvm.so+0x17eb625] VMThread::run()+0x107 (vmThread.cpp:175)
V [libjvm.so+0x16c666f] Thread::call_run()+0x1bd (thread.cpp:217)
V [libjvm.so+0x1325e86] thread_native_entry(Thread*)+0x1cf (os_linux.cpp:783)
Registers:

```