@Override
    public boolean allocate(int oldOffset, int newOffset) {
      if (!allocateCalled) {
        allocateCalled = true;
        return false;
      }
      return super.allocate(oldOffset, newOffset);
    }
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