string_list_append: always set util pointer to NULL

It is not immediately obvious that the util field may contain random bytes after appending an item. Especially since the string_list_insert* functions _do_ explicitly zero the util pointer. This does not appear to be a bug in any current git code, as all callers either fill in the util field immediately or never use it. However, it is worth it to be less surprising to new users of the string-list API who may expect it to be intialized to NULL. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>