Fix a crash when closing an X11 window with a selection

This was caused by weston_wm_handle_xfixes_selection_notify() calling
weston_seat_set_selection() with a NULL source, apparently only
sometimes when closing an Xwayland window.

libweston/data-device.c

@@ -1152,7 +1152,9 @@ weston_seat_set_selection(struct weston_seat *seat,
 
 	seat->selection_data_source = source;
 	seat->selection_serial = serial;
-	source->set_selection = true;
+
+	if (source)
+		source->set_selection = true;
 
 	if (keyboard)
 		focus = keyboard->focus;