@Override
    protected OffHeapStoredObjectAddressStack createFreeListForEmptySlot(
        AtomicReferenceArray<OffHeapStoredObjectAddressStack> freeLists, int idx) {
      if (firstTime) {
        firstTime = false;
        OffHeapStoredObjectAddressStack clq = super.createFreeListForEmptySlot(freeLists, idx);
        if (!freeLists.compareAndSet(idx, null, clq)) {
          fail("this should never happen. Indicates a concurrent modification");
        }
      }
      return super.createFreeListForEmptySlot(freeLists, idx);
    }
Just like Fragment except that the first time allocate is called it returns false indicating
that the allocate failed. In a real system this would only happen if a concurrent allocate
happened. This allows better code coverage.
geode-core/src/test/java/org/apache/geode/internal/offheap/FreeListManagerTest.java