add test

Author: devnexen

ext/gd/tests/gh16771.phpt

@@ -0,0 +1,10 @@
+--TEST--
+GH-16771 (UBSan abort in ext/gd/libgd/gd.c:1372)
+--EXTENSIONS--
+gd
+--FILE--
+<?php
+$string_mb = base64_decode('5pel5pys6Kqe44OG44Kt44K544OIMzTvvJXvvJbml6XmnKzoqp7jg4bjgq3jgrnjg4g=');
+imagecreatefromstring($string_mb);
+--EXPECTF--
+Warning: imagecreatefromstring(): Data is not in a recognized format in %s on line %d