delta-islands: fix segfault when freeing island marks

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.

The root cause of this bug is that we unconditionally dereference the island_marks variable even in the case where it is a NULL pointer, which is fixed by making it conditional. Note that we still leave the logic in place to set the pointer to -1 to detect use-after-free bugs even when there are no loaded island marks at all.

Fixes #146 (closed).