Fix S3 GetChildren() substr index error

[annie-anna]

Found a bug in S3 GetChildren, which will give substr index error.

int GetChildren(const TF_Filesystem* filesystem, const char* path,
                char*** entries, TF_Status* status) {
  TF_VLog(1, "GetChildren for path: %s\n", path);
  Aws::String bucket, prefix;
  ParseS3Path(path, true, &bucket, &prefix, status);
  if (TF_GetCode(status) != TF_OK) return -1;
  if (!prefix.empty() && prefix.back() != '/') prefix.push_back('/');    // for example, prefix='dir1/'

  auto s3_file = static_cast<S3File*>(filesystem->plugin_filesystem);
  GetS3Client(s3_file);

  Aws::S3::Model::ListObjectsV2Request list_objects_request;
  list_objects_request.WithBucket(bucket)
      .WithPrefix(prefix)
      .WithMaxKeys(kS3GetChildrenMaxKeys)
      .WithDelimiter("/");
  list_objects_request.SetResponseStreamFactory(
      []() { return Aws::New<Aws::StringStream>(kS3FileSystemAllocationTag); });

  Aws::S3::Model::ListObjectsV2Result list_objects_result;
  std::vector<Aws::String> result;
  do {
    auto list_objects_outcome =
        s3_file->s3_client->ListObjectsV2(list_objects_request);
    if (!list_objects_outcome.IsSuccess()) {
      TF_SetStatusFromAWSError(list_objects_outcome.GetError(), status);
      return -1;
    }

    list_objects_result = list_objects_outcome.GetResult();
    for (const auto& object : list_objects_result.GetCommonPrefixes()) {  // 'CommonPrefixes': [{'Prefix': 'dir1/'}]
      Aws::String s = object.GetPrefix();                                 // object={'Prefix': 'dir1/'}, s='dir1/'
      s.erase(s.length() - 1);                                            // s='dir1'
      Aws::String entry = s.substr(prefix.length());                      // s='dir1', prefix='dir1/', prefix.length() > s.length()
      if (entry.length() > 0) {
        result.push_back(entry);
      }
    }
    for (const auto& object : list_objects_result.GetContents()) {
      Aws::String s = object.GetKey();
      Aws::String entry = s.substr(prefix.length());
      if (entry.length() > 0) {
        result.push_back(entry);
      }
    }
    list_objects_request.SetContinuationToken(
        list_objects_result.GetNextContinuationToken());
  } while (list_objects_result.GetIsTruncated());

  int num_entries = result.size();
  *entries = static_cast<char**>(
      plugin_memory_allocate(num_entries * sizeof((*entries)[0])));
  for (int i = 0; i < num_entries; i++)
    (*entries)[i] = strdup(result[i].c_str());
  TF_SetStatus(status, TF_OK, "");
  return num_entries;
}

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[annie-anna]

@mihaimaruseac @yongtang Could you review the PR?