Validate that filename corresponds to labels for the MNIST dataset.

def check_labels_file_header(filename):
  """Validate that filename corresponds to labels for the MNIST dataset."""
  with tf.gfile.Open(filename, 'rb') as f:
    magic = read32(f)
    read32(f)  # num_items, unused
    if magic != 2049:
      raise ValueError('Invalid magic number %d in MNIST file %s' % (magic,
                                                                     f.name))