object-name: explicitly handle bad tags in show_ambiguous_object()

object-name: explicitly handle bad tags in show_ambiguous_object()

Follow-up the handling of OBJ_BAD in the preceding commit and
explicitly handle those cases where parse_tag() fails, or we don't end
up with a non-NULL pointer in in tag->tag.

If we run into such a tag we'd previously be silent about it. We
really should also be handling these batter in parse_tag_buffer() by
being more eager to emit an error(), instead of silently aborting with
"return -1;".

One example of such a tag is the one that's tested for in
"t3800-mktag.sh", where the code takes the "size <
the_hash_algo->hexsz + 24" branch.

But in lieu of earlier missing "error" output let's show the user
something to indicate why we're not showing a tag message in these
cases, now instead of showing:

    hint:   deadbeef tag

We'll instead display:

    hint:   deadbeef tag [tag could not be parsed]

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>