diffstat: do not rely on uninitialized data

Right now if you visit:
<http://git.zx2c4.com/systemd/diff/src/udev/udev-builtin-input_id.c?id=bcfce235>
you'll see that if you reload the page a few times, a bunch of times the
diffstat comes out with no lines being shown or changed. I'm not
currently sure what the cause of this is, but I suspect it might have to
do with this uninitialized data.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>