glx: Be more strict about rejecting invalid image sizes [CVE-2014-8093 2/6]

Before this we'd just clamp the image size to 0, which was just
hideously stupid; if the parameters were such that they'd overflow an
integer, you'd allocate a small buffer, then pass huge values into (say)
ReadPixels, and now you're scribbling over arbitrary server memory.

Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Reviewed-by: Michal Srb <msrb@suse.com>
Reviewed-by: Andy Ritger <aritger@nvidia.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>