shuf: exit without reading if would never output

[Pádraig Brady]

Similar to the tail -n0 patch recently sent to the list,
this adjusts shuf so that it never reads input with -n0.

Now unlike tail -n0 which is useful with -f, shuf -n0
isn't needed, as it always truncates --output if specified,
and doesn't produce anything.
I don't think we should disallow -n0 though, as it could validly
be used in scripts as a concise way to produce 0..N outputs.

thanks,
Pádraig