screen-share: don't test keyboard/pointer pointers

screen-share: don't test keyboard/pointer pointers

Keyboards and pointers aren't freed when devices are removed, so we
should really be testing keyboard_device_count and pointer_device_count
in most cases, not the actual pointers. Otherwise we end up with
different behaviour after removing a device than we had before it was
inserted.

This commit makes screen-share.c compile once again after changes in
commit 1281a36e3bcd27345bd4a107f282213ecca56f0e.

Signed-off-by: Dawid Gajownik <gajownik@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>