xwayland/selection: make `xwm_data_source_write` return 0 on failure

The `fd` is marked `O_NONBLOCK`, so `write` will never spuriously return
`EINTR`. Therefore, `write` failing is permanent, and we can return 0 to
make the return value meaningful.

xwayland/selection/incoming.c

@@ -28,7 +28,7 @@ static int xwm_data_source_write(int fd, uint32_t mask, void *data) {
 		xwm_selection_transfer_destroy_property_reply(transfer);
 		xwm_selection_transfer_remove_event_source(transfer);
 		xwm_selection_transfer_close_wl_client_fd(transfer);
-		return 1;
+		return 0;
 	}
 
 	wlr_log(WLR_DEBUG, "wrote %zd (chunk size %zd) of %d bytes",