HID: wacom: NULL dereferences on error in probe()

We can't pass a NULL to input_unregister_device().

Fixes: 2a6cdbdd ('HID: wacom: Introduce new 'touch_input' device')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_sys.c

@@ -1271,11 +1271,13 @@ static int wacom_register_inputs(struct wacom *wacom)
 	pad_input_dev = NULL;
 	wacom_wac->pad_registered = false;
 fail_register_pad_input:
-	input_unregister_device(touch_input_dev);
+	if (touch_input_dev)
+		input_unregister_device(touch_input_dev);
 	wacom_wac->touch_input = NULL;
 	wacom_wac->touch_registered = false;
 fail_register_touch_input:
-	input_unregister_device(pen_input_dev);
+	if (pen_input_dev)
+		input_unregister_device(pen_input_dev);
 	wacom_wac->pen_input = NULL;
 	wacom_wac->pen_registered = false;
 fail_register_pen_input: