parse_tag_buffer(): treat NULL tag pointer as parse error

parse_tag_buffer(): treat NULL tag pointer as parse error

When parsing a tag, we may end up with a NULL "tagged" field when
there's a type mismatch (e.g., the tag claims to point to object X as a
commit, but we previously saw X as a blob in the same process), but we
do not otherwise indicate a parse failure to the caller.

This is similar to the case discussed in the previous commit, where a
commit could end up with a NULL tree field: while slightly convenient
for callers who want to overlook a corrupt object, it means that normal
callers have to explicitly deal with this case (rather than just relying
on the return code from parsing). And most don't, leading to segfault
fixes like the one in c77722b3ea (use get_tagged_oid(), 2019-09-05).

Let's address this more centrally, by returning an error code from the
parse itself, which most callers would already notice (adventurous
callers are free to ignore the error and continue looking at the
struct).

This also covers the case where the tag contains a nonsensical "type"
field (there we produced a user-visible error but still returned success
to the caller; now we'll produce a slightly better message and return an
error).

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