If the size computation routine returns -1 we should just reject the
request outright.  Clamping it to zero could give an attacker the
opportunity to also mangle cmdlen in such a way that the subsequent
length check passes, and the request would get executed, thus passing
data we wanted to reject to the renderer.

v3: backport to nx-libs 3.6.x (Mike DePaulo)
v2: backport to RHEL5 - fix swap paths
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>
Signed-off-by: Fedora X Ninjas <x@fedoraproject.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

fixup swaps