xwayland/selection: end incr transfer on empty prop, not next selection

Previously, `transfer->incr` was being cleared on the next selection.
However, if the next selection was *also* incremental, it's possible
that `xwm_handle_selection_property_notify` would route us to
`xwm_get_incr_chunk` instead of `xwm_selection_get_data`.

xwayland/selection/selection.c

@@ -35,6 +35,7 @@ void xwm_selection_transfer_destroy_property_reply(
 
 void xwm_selection_transfer_finish(
 		struct wlr_xwm_selection_transfer *transfer) {
+	transfer->incr = false;
 	xwm_selection_transfer_destroy_property_reply(transfer);
 	xwm_selection_transfer_remove_event_source(transfer);
 	xwm_selection_transfer_close_wl_client_fd(transfer);