xwayland: Check the condition on displayfd ready

The callback on_displayfd_ready() would unconditionally set the return
value to TRUE, regardless of the condition.

Use the GIO condition to determine if there was data written and adjust
the return value accordingly.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1851>

src/wayland/meta-xwayland.c

@@ -898,7 +898,7 @@ on_displayfd_ready (int          fd,
    * socket when it's ready. We don't care about the data
    * in the socket, just that it wrote something, since
    * that means it's ready. */
-  g_task_return_boolean (task, TRUE);
+  g_task_return_boolean (task, !!(condition & G_IO_IN));
   g_object_unref (task);
 
   return G_SOURCE_REMOVE;