checkout, clone: die if tree cannot be parsed

When a tree oid is invalid, parse_tree_indirect() can return NULL. Check
for NULL instead of proceeding as though it were a valid pointer and
segfaulting.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -733,6 +733,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		struct tree_desc trees[2];
 		struct tree *tree;
 		struct unpack_trees_options topts;
+		const struct object_id *old_commit_oid;
 
 		memset(&topts, 0, sizeof(topts));
 		topts.head_idx = -1;
@@ -760,9 +761,15 @@ static int merge_working_tree(const struct checkout_opts *opts,
 				       &new_branch_info->commit->object.oid :
 				       &new_branch_info->oid, NULL);
 		topts.preserve_ignored = !opts->overwrite_ignore;
-		tree = parse_tree_indirect(old_branch_info->commit ?
-					   &old_branch_info->commit->object.oid :
-					   the_hash_algo->empty_tree);
+
+		old_commit_oid = old_branch_info->commit ?
+			&old_branch_info->commit->object.oid :
+			the_hash_algo->empty_tree;
+		tree = parse_tree_indirect(old_commit_oid);
+		if (!tree)
+			die(_("unable to parse commit %s"),
+				oid_to_hex(old_commit_oid));
+
 		init_tree_desc(&trees[0], tree->buffer, tree->size);
 		parse_tree(new_tree);
 		tree = new_tree;

builtin/clone.c

@@ -695,6 +695,8 @@ static int checkout(int submodule_progress)
 	init_checkout_metadata(&opts.meta, head, &oid, NULL);
 
 	tree = parse_tree_indirect(&oid);
+	if (!tree)
+		die(_("unable to parse commit %s"), oid_to_hex(&oid));
 	parse_tree(tree);
 	init_tree_desc(&t, tree->buffer, tree->size);
 	if (unpack_trees(1, &t, &opts) < 0)