Reduce signed/unsigned comparison warnings

core/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp

 // Copyright 2014 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_page/pageint.h"
 
 #include <limits.h>
 
@@ skipping 30 matching lines @@
 CPDF_StreamParser::~CPDF_StreamParser() {
   if (m_pLastObj) {
     m_pLastObj->Release();
   }
 }
 
 FX_DWORD DecodeAllScanlines(ICodec_ScanlineDecoder* pDecoder,
                             uint8_t*& dest_buf,
                             FX_DWORD& dest_size) {
   if (!pDecoder) {
-    return static_cast<FX_DWORD>(-1);
+    return FX_INVALID_OFFSET;
   }
   int ncomps = pDecoder->CountComps();
   int bpc = pDecoder->GetBPC();
   int width = pDecoder->GetWidth();
   int height = pDecoder->GetHeight();
   int pitch = (width * ncomps * bpc + 7) / 8;
   if (height == 0 || pitch > (1 << 30) / height) {
     delete pDecoder;
-    return static_cast<FX_DWORD>(-1);
+    return FX_INVALID_OFFSET;
   }
   dest_buf = FX_Alloc2D(uint8_t, pitch, height);
   dest_size = pitch * height;  // Safe since checked alloc returned.
   for (int row = 0; row < height; row++) {
     const uint8_t* pLine = pDecoder->GetScanline(row);
     if (!pLine)
       break;
 
     FXSYS_memcpy(dest_buf + row * pitch, pLine, pitch);
   }
@@ skipping 814 matching lines @@
       }
       m_Status = Done;
       return;
     }
     steps++;
     if (pPause && pPause->NeedToPauseNow()) {
       break;
     }
   }
 }