Fix the code that causes warnings

xfa/fgas/xml/fgas_sax.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 "xfa/fgas/xml/fgas_sax.h"
 
 #include <algorithm>
 
@@ skipping 14 matching lines @@
       m_dwBufSize(0),
       m_dwBufIndex(0) {}
 FX_BOOL CFX_SAXFile::StartFile(IFX_FileRead* pFile,
                                uint32_t dwStart,
                                uint32_t dwLen) {
   ASSERT(m_pFile == NULL && pFile != NULL);
   uint32_t dwSize = pFile->GetSize();
   if (dwStart >= dwSize) {
     return FALSE;
   }
-  if (dwLen == -1 || dwStart + dwLen > dwSize) {
+  if (dwLen == static_cast<uint32_t>(-1) || dwStart + dwLen > dwSize) {
     dwLen = dwSize - dwStart;
   }
   if (dwLen == 0) {
     return FALSE;
   }
   m_dwBufSize = std::min(dwLen, kSaxFileBufSize);
   m_pBuf = FX_Alloc(uint8_t, m_dwBufSize);
   if (!pFile->ReadBlock(m_pBuf, dwStart, m_dwBufSize)) {
     return FALSE;
   }
@@ skipping 633 matching lines @@
 void CFX_SAXReader::SkipCurrentNode() {
   if (!m_pCurItem)
     return;
 
   m_pCurItem->m_bSkip = TRUE;
 }
 
 void CFX_SAXReader::SetHandler(CXFA_SAXReaderHandler* pHandler) {
   m_pHandler = pHandler;
 }