xz: Avoid arithmetic on a null pointer

xz: Avoid arithmetic on a null pointer

It's undefined behavior. The result wasn't ever used as it occurred
in the last iteration of a loop.

Clang 17 with -fsanitize=address,undefined:

    $ src/xz/xz --block-list=123
    src/xz/args.c:164:12: runtime error: applying non-zero offset 1
        to null pointer

Fixes: 88ccf47205d7f3aa314d358c72ef214f10f68b43
Co-authored-by: Sam James <sam@gentoo.org>
(cherry picked from commit 77c8f60547decefca8f2d0c905d9c708c38ee8ff)