Grabbing a disabled (pointer) device will lead to a segfault later
in the myriad of places where we look at the device's spriteInfo - which
will be NULL.

As a workaround, disallow grabbing a disabled device by pretending it's
already grabbed. Since the point of a grab is to receive all events by
that device and disabled devices cannot send events, this should be Good
Enough.

Tested-by: Olivier Fourdan <ofourdan@redhat.com>
Part-of: <!1856>