merge-ort: do check parse_tree()'s return value

The previous commit fixed a bug where a missing tree was reported, but
not treated as an error.

This patch addresses the same issue for the remaining two callers of
`parse_tree()`.

This change is not accompanied by a regression test because the code in
question is only reached at the `checkout` stage, i.e. after the merge
has happened (and therefore the tree objects could only be missing if
the disk had gone bad in that short time window, or something similarly
tricky to recreate in the test suite).

Signed-off-by: Johannes Schindelin <[email protected]>

merge-ort.c

@@ -4379,9 +4379,11 @@ static int checkout(struct merge_options *opt,
 	unpack_opts.verbose_update = (opt->verbosity > 2);
 	unpack_opts.fn = twoway_merge;
 	unpack_opts.preserve_ignored = 0; /* FIXME: !opts->overwrite_ignore */
-	parse_tree(prev);
+	if (parse_tree(prev) < 0)
+		return -1;
 	init_tree_desc(&trees[0], prev->buffer, prev->size);
-	parse_tree(next);
+	if (parse_tree(next) < 0)
+		return -1;
 	init_tree_desc(&trees[1], next->buffer, next->size);
 
 	ret = unpack_trees(2, trees, &unpack_opts);