Fix GetCareMap for small images

  disable_sparse = OPTIONS.info_dict.get(which + "_disable_sparse")

  image_blocks = int(image_size) // 4096 - 1

  assert image_blocks > 0, "blocks for {} must be positive".format(which)

  # It's OK for image_blocks to be 0, because care map ranges are inclusive.

  # So 0-0 means "just block 0", which is valid.

  assert image_blocks >= 0, "blocks for {} must be non-negative, image size: {}".format(

      which, image_size)

  # For sparse images, we will only check the blocks that are listed in the care

  # map, i.e. the ones with meaningful data.