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>