Fix memory leak

Fixes oss-fuzz #51451

Author: dstogov

Zend/tests/gc_046.phpt

@@ -0,0 +1,23 @@
+--TEST--
+GC 046: Leak in User Iterator
+--INI--
+zend.enable_gc=1
+--FILE--
+<?php
+class Action {
+    function __construct() {
+        $this->iterator = new ArrayIterator($this);
+    }
+    function filter() {
+        $this->iterator = new CallbackFilterIterator($this->iterator, fn() => true);
+        $this->iterator->rewind();
+    }
+}
+
+$action=new Action;
+$action->filter();
+$action->filter();
+?>
+DONE
+--EXPECT--
+DONE

Zend/zend_interfaces.c

@@ -185,8 +185,15 @@ ZEND_API void zend_user_it_rewind(zend_object_iterator *_iter)
 ZEND_API HashTable *zend_user_it_get_gc(zend_object_iterator *_iter, zval **table, int *n)
 {
 	zend_user_iterator *iter = (zend_user_iterator*)_iter;
-	*table = &iter->it.data;
-	*n = 1;
+	if (Z_ISUNDEF(iter->value)) {
+		*table = &iter->it.data;
+		*n = 1;
+	} else {
+		zend_get_gc_buffer *gc_buffer = zend_get_gc_buffer_create();
+		zend_get_gc_buffer_add_zval(gc_buffer, &iter->it.data);
+		zend_get_gc_buffer_add_zval(gc_buffer, &iter->value);
+		zend_get_gc_buffer_use(gc_buffer, table, n);
+	}
 	return NULL;
 }