test_bitmap_hashes(): handle repository without bitmaps

test_bitmap_hashes(): handle repository without bitmaps

If prepare_bitmap_git() returns NULL (one easy-to-trigger cause being
that the repository does not have bitmaps at all), then we'll segfault
accessing bitmap_git->hashes:

  $ t/helper/test-tool bitmap dump-hashes
  Segmentation fault

We should treat this the same as a repository with bitmaps but no
name-hashes, and quietly produce an empty output. The later call to
free_bitmap_index() in the cleanup label is OK, as it treats a NULL
pointer as a noop.

This isn't a big deal in practice, as this function is intended for and
used only by test-tool. It's probably worth fixing to avoid confusion,
but not worth adding coverage for this to the test suite.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>