merge-ort: clean up after failed merge

In 9fefce68dc (merge-ort: basic outline for merge_switch_to_result(),
2020-12-13), we added functionality to lay down the result of a merge on
disk. But we forgot to release the data structures in case
`unpack_trees()` failed to run properly.

This was pointed out by the `linux-leaks` job in our CI runs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort.c

@@ -4225,6 +4225,7 @@ void merge_switch_to_result(struct merge_options *opt,
 		if (checkout(opt, head, result->tree)) {
 			/* failure to function */
 			result->clean = -1;
+			merge_finalize(opt, result);
 			return;
 		}
 		trace2_region_leave("merge", "checkout", opt->repo);
@@ -4235,6 +4236,7 @@ void merge_switch_to_result(struct merge_options *opt,
 			/* failure to function */
 			opt->priv = NULL;
 			result->clean = -1;
+			merge_finalize(opt, result);
 			return;
 		}
 		opt->priv = NULL;