Reduce signed/unsigned comparison warnings

core/fpdfapi/fpdf_parser/fpdf_parser_decode.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_parser/include/fpdf_parser_decode.h"
 
 #include <limits.h>
 #include <vector>
@@ skipping 158 matching lines @@
                          FX_DWORD src_size,
                          uint8_t*& dest_buf,
                          FX_DWORD& dest_size) {
   FX_DWORD i = 0;
   FX_DWORD old;
   dest_size = 0;
   while (i < src_size) {
     if (src_buf[i] < 128) {
       old = dest_size;
       dest_size += src_buf[i] + 1;
-      if (dest_size < old) {
-        return static_cast<FX_DWORD>(-1);
-      }
+      if (dest_size < old)
+        return FX_INVALID_OFFSET;
       i += src_buf[i] + 2;
     } else if (src_buf[i] > 128) {
       old = dest_size;
       dest_size += 257 - src_buf[i];
-      if (dest_size < old) {
-        return static_cast<FX_DWORD>(-1);
-      }
+      if (dest_size < old)
+        return FX_INVALID_OFFSET;
       i += 2;
     } else {
       break;
     }
   }
-  if (dest_size >= _STREAM_MAX_SIZE_) {
-    return static_cast<FX_DWORD>(-1);
-  }
+  if (dest_size >= _STREAM_MAX_SIZE_)
+    return FX_INVALID_OFFSET;
   dest_buf = FX_Alloc(uint8_t, dest_size);
   i = 0;
   int dest_count = 0;
   while (i < src_size) {
     if (src_buf[i] < 128) {
       FX_DWORD copy_len = src_buf[i] + 1;
       FX_DWORD buf_left = src_size - i - 1;
       if (buf_left < copy_len) {
         FX_DWORD delta = copy_len - buf_left;
         copy_len = buf_left;
@@ skipping 376 matching lines @@
                      FX_DWORD src_size,
                      uint8_t*& dest_buf,
                      FX_DWORD& dest_size) {
   CCodec_ModuleMgr* pEncoders = CPDF_ModuleMgr::Get()->GetCodecModule();
   if (pEncoders) {
     return pEncoders->GetFlateModule()->FlateOrLZWDecode(
         FALSE, src_buf, src_size, FALSE, 0, 0, 0, 0, 0, dest_buf, dest_size);
   }
   return 0;
 }