get_commit_tree(): return NULL for broken tree

Return NULL from 'get_commit_tree()' when a commit's root tree is
corrupt, doesn't exist, or points to an object which is not a tree.

In [1], this situation became a BUG(), but it can certainly occur in
cases which are not a bug in Git, for e.g., if a caller manually crafts
a commit whose tree is corrupt in any of the above ways.

Note that the expect_failure test in t6102 triggers this BUG(), but we
can't flip it to expect_success yet. Solving this problem actually
reveals a second bug.

[1]: 7b8a21dba1 (commit-graph: lazy-load trees for commits, 2018-04-06)

Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -345,10 +345,10 @@ struct tree *get_commit_tree(const struct commit *commit)
 	if (commit->maybe_tree || !commit->object.parsed)
 		return commit->maybe_tree;
 
-	if (commit->graph_pos == COMMIT_NOT_FROM_GRAPH)
-		BUG("commit has NULL tree, but was not loaded from commit-graph");
+	if (commit->graph_pos != COMMIT_NOT_FROM_GRAPH)
+		return get_commit_tree_in_graph(the_repository, commit);
 
-	return get_commit_tree_in_graph(the_repository, commit);
+	return NULL;
 }
 
 struct object_id *get_commit_tree_oid(const struct commit *commit)