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>

drivers/hid/wacom_wac.c

@@ -2548,6 +2548,9 @@ static void wacom_wac_finger_slot(struct wacom_wac *wacom_wac,
 		int slot;
 
 		slot = input_mt_get_slot_by_key(input, hid_data->id);
+		if (slot < 0)
+			return;
+
 		input_mt_slot(input, slot);
 		input_mt_report_slot_state(input, MT_TOOL_FINGER, prox);
 	}