HID: wacom: Skip processing of touches with negative slot values

The `input_mt_get_slot_by_key` function may return a negative value
if an error occurs (e.g. running out of slots). If this occurs we
should really avoid reporting any data for the slot.

Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>