Prevent theoretical null-pointer dereference

Cannot happen since we don't enter this function if the atom isn't one
of the three checked above but the static analyzer doesn't know that.

Part-of: <https://gitlab.freedesktop.org/xorg/driver/xf86-input-libinput/-/merge_requests/60>

src/xf86libinput.c

@@ -4678,6 +4678,8 @@ LibinputSetPropertyAccelPoints(DeviceIntPtr dev,
 			accel_points = &driver_data->options.accel_points_motion;
 		else if (atom == prop_accel_points_scroll)
 			accel_points = &driver_data->options.accel_points_scroll;
+		else
+			return BadValue;
 
 		for (size_t idx = 0; idx < val->size; idx++)
 			accel_points->points[idx] = data[idx];