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

name-hash.c

@@ -74,7 +74,7 @@ static void hash_index_entry(struct index_state *istate, struct cache_entry *ce)
 	if (ce->ce_flags & CE_HASHED)
 		return;
 	ce->ce_flags |= CE_HASHED;
-	ce->next = NULL;
+	ce->next = ce->dir_next = NULL;
 	hash = hash_name(ce->name, ce_namelen(ce));
 	pos = insert_hash(hash, ce, &istate->name_hash);
 	if (pos) {