[settings] Correctly mortalise autovivified child settings blocks

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

 				&new_child->autovivified.refcnt );
 	settings = &new_child->autovivified.generic.settings;
 	register_settings ( settings, parent, new_child->name );
+	ref_put ( settings->refcnt );
 	return settings;
 }