Fix check in user filter dtor

obj is always non-null, we need to check for UNDEF here.

This was caught by the new assertion introduced in
3ec1400.

Author: nikic

ext/standard/user_filters.c

@@ -114,7 +114,7 @@ static void userfilter_dtor(php_stream_filter *thisfilter)
 	zval func_name;
 	zval retval;
 
-	if (obj == NULL) {
+	if (Z_ISUNDEF_P(obj)) {
 		/* If there's no object associated then there's nothing to dispose of */
 		return;
 	}