dix: avoid deferencing NULL PtrCtrl

PtrCtrl really makes sense for relative pointing device only, absolute
devices such as touch devices do not have any PtrCtrl set.

In some cases, if the client issues a XGetPointerControl() immediatlely
after a ChangeMasterDeviceClasses() copied the touch device to the VCP,
a NULL pointer dereference will occur leading to a crash of Xwayland.

Check whether the PtrCtrl is not NULL in ProcGetPointerControl() and
return the default control values otherwise, to avoid the NULL pointer
dereference.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1519533


Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit 9f7a9be1)