Don't allow properties on GdImage

Just like all the other former resources... we missed this case.

Author: nikic

ext/gd/gd.c

@@ -222,7 +222,7 @@ static void php_gd_object_minit_helper()
 	zend_class_entry ce;
 	INIT_CLASS_ENTRY(ce, "GdImage", class_GdImage_methods);
 	gd_image_ce = zend_register_internal_class(&ce);
-	gd_image_ce->ce_flags |= ZEND_ACC_FINAL;
+	gd_image_ce->ce_flags |= ZEND_ACC_FINAL | ZEND_ACC_NO_DYNAMIC_PROPERTIES;
 	gd_image_ce->create_object = php_gd_image_object_create;
 	gd_image_ce->serialize = zend_class_serialize_deny;
 	gd_image_ce->unserialize = zend_class_unserialize_deny;