parse_commit_buffer(): treat lookup_tree() failure as parse error

If parsing a commit yields a valid tree oid, but we've seen that same
oid as a non-tree in the same process, the resulting commit struct will
end up with a NULL tree pointer, but not otherwise report a parsing
failure.

That's perhaps convenient for callers which want to operate on even
partially corrupt commits (e.g., by still looking at the parents). But
it leaves a potential trap for most callers, who now have to manually
check for a NULL tree. Most do not, and it's likely that there are
possible segfaults lurking. I say "possible" because there are many
candidates, and I don't think it's worth following through on
reproducing them when we can just fix them all in one spot. And
certainly we _have_ seen real-world cases, such as the one fixed by
806278dead (commit-graph.c: handle corrupt/missing trees, 2019-09-05).

Note that we can't quite drop the check in the caller added by that
commit yet, as there's some subtlety with repeated parsings (which will
be addressed in a future commit).

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

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