Segfault when freeing island marks without loaded delta islands

In 647982bb (delta-islands: free island_marks and bitmaps, 2023-02-03) we have introduced logic to free island_marks in order to reduce heap memory usage in git-pack-objects(1). This commit is causing segfaults in the case where this Git command does not load delta islands at all, e.g. when reading object IDs from standard input. One such crash can be hit when using repacking multi-pack-indices with delta islands enabled.