fix infinite loops when rebuild the cross reference table

core/fpdfapi/fpdf_parser/cpdf_parser.cpp

 // Copyright 2016 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.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "core/fpdfapi/fpdf_parser/include/cpdf_parser.h"
 
 #include <vector>
 
@@ skipping 913 matching lines @@
           break;
       }
 
       if (bOverFlow) {
         size = 0;
         break;
       }
     }
     pos += size;
 
-    // If the position has not changed at all in a loop iteration, then break
-    // out to prevent infinite looping.
-    if (pos == saved_pos)
+    // If the position has not changed at all or went backwards in a loop
+    // iteration, then break out to prevent infinite looping.
+    if (pos <= saved_pos)
       break;
   }
 
   if (last_xref != -1 && last_xref > last_obj)
     last_trailer = last_xref;
   else if (last_trailer == -1 || last_xref < last_obj)
     last_trailer = m_pSyntax->m_FileLen;
 
   m_SortedOffset.insert(last_trailer - m_pSyntax->m_HeaderOffset);
   return m_pTrailer && !m_ObjectInfo.empty();
@@ skipping 697 matching lines @@
   if (!LoadLinearizedAllCrossRefV4(m_LastXRefOffset, m_dwXrefStartObjNum) &&
       !LoadLinearizedAllCrossRefV5(m_LastXRefOffset)) {
     m_LastXRefOffset = 0;
     m_pSyntax->m_MetadataObjnum = dwSaveMetadataObjnum;
     return FORMAT_ERROR;
   }
 
   m_pSyntax->m_MetadataObjnum = dwSaveMetadataObjnum;
   return SUCCESS;
 }