Fix infinite loop for bookmark search

fpdfsdk/src/fpdfdoc_embeddertest.cpp

 // Copyright 2015 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/include/fxcrt/fx_string.h"
 #include "public/fpdf_doc.h"
 #include "public/fpdfview.h"
 #include "testing/embedder_test.h"
 #include "testing/fx_string_testhelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 107 matching lines @@
             CFX_WideString::FromUTF16LE(buf, 16));
 
   // Check that it is them same as the one returned by GetFirstChild.
   EXPECT_EQ(child, FPDFBookmark_GetFirstChild(document(), nullptr));
 
   // Try to find one using a non-existent title.
   std::unique_ptr<unsigned short, pdfium::FreeDeleter> bad_title =
       GetFPDFWideString(L"A BAD Beginning");
   EXPECT_EQ(nullptr, FPDFBookmark_Find(document(), bad_title.get()));
 }
+
+// Check circular bookmarks will not cause infinite loop.
+TEST_F(FPDFDocEmbeddertest, FindBookmarks_bug420) {
+  // Open a file with circular bookmarks.
+  EXPECT_TRUE(OpenDocument("bookmarks_circular.pdf"));
+
+  // Try to find a title.
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> title =
+      GetFPDFWideString(L"anything");
+  EXPECT_EQ(nullptr, FPDFBookmark_Find(document(), title.get()));
+}