Fix segfault when unserializing abstract class

Author: nikic

ext/standard/tests/serialize/unserialize_abstract_class.phpt

@@ -0,0 +1,15 @@
+--TEST--
+Unserializing an abstract class should fail
+--FILE--
+<?php
+
+$payload = 'O:23:"RecursiveFilterIterator":0:{}';
+try {
+    var_dump(unserialize($payload));
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+
+?>
+--EXPECT--
+Cannot instantiate abstract class RecursiveFilterIterator

ext/standard/var_unserializer.re

@@ -1144,7 +1144,11 @@ object ":" uiv ":" ["]	{
 		return 0;
 	}
 
-	object_init_ex(rval, ce);
+	if (object_init_ex(rval, ce) == FAILURE) {
+		zend_string_release_ex(class_name, 0);
+		return 0;
+	}
+
 	if (incomplete_class) {
 		php_store_class_name(rval, ZSTR_VAL(class_name), len2);
 	}