libgimpwidgets: slightly better form.

Previous code was right and equivalent because
gdk_wayland_window_export_handle() works asynchronously in the same
thread (so phandle can't get overridden by mistake). Nevertheless a
quick code scan felt surprising, so to avoid any such future surprise,
let's just switch the order of statements.

libgimpwidgets/gimpwidgetsutils.c

@@ -1362,6 +1362,9 @@ gimp_widget_set_handle_on_mapped (GtkWidget    *widget,
       handle = g_bytes_new (&id, sizeof (Window));
     }
 #endif
+
+  *phandle = handle;
+
 #ifdef GDK_WINDOWING_WAYLAND
   if (GDK_IS_WAYLAND_WINDOW (surface))
     {
@@ -1379,7 +1382,5 @@ gimp_widget_set_handle_on_mapped (GtkWidget    *widget,
     }
 #endif
 
-  *phandle = handle;
-
   return FALSE;
 }