randr: make randr code not segfault when xinerama set

[Dave Airlie]

On Nov 30, 2007 3:18 AM, Keith Packard <keithp at keithp.com> wrote:
> +    if (!xf86RandR12Index)
> +       return;
>
> seems inconsistent with the change which sets xf86RandR12Index to -1
> when RandR is not running.

The follow-up patch fixes all the -1 checks.. or at least it  should have..

I didn't realise 0 was a valid index the first time..

Dave.