Add regression test for GH-21824

Author: prateekbhujel

ext/standard/tests/serialize/gh21824.phpt

@@ -0,0 +1,24 @@
+--TEST--
+GH-21824 (Crash in unserialize() with destructor reentry)
+--ENV--
+USE_ZEND_ALLOC=0
+--FILE--
+<?php
+class Trigger {
+    public function __destruct() {
+        unserialize('i:1;');
+    }
+}
+
+$payload = 'a:1019:{';
+for ($i = 0; $i < 1017; $i++) {
+    $payload .= "i:$i;s:1:\"A\";";
+}
+$payload .= 'i:1017;O:7:"Trigger":0:{}';
+$payload .= 'i:1017;i:0;}';
+
+unserialize($payload);
+echo "Done\n";
+?>
+--EXPECT--
+Done