Try replacing

ASSERT(lost_plus_found_directory_iter != all_directories.end());

with

  if (lost_plus_found_directory_iter == all_directories.end())
  {
    --lost_plus_found_directory_iter;
    std::cout << "WARNING: Cannot find lost+find directory?! Stuffing unlinkable blocks in " << lost_plus_found_directory_iter->first << " instead./n";
  }