name-hash.c: always initialize dir_next pointer

name-hash.c: always initialize dir_next pointer

Test t2021-checkout-overwrite.sh reveals a segfault in 'git add' on a
case-insensitive file system when git is compiled with XMALLOC_POISON
defined. The reason is that 2548183b (fix phantom untracked files when
core.ignorecase is set) added a new member dir_next to struct cache_entry,
but forgot to initialize it in all cases.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>