Fix and integer overflow issue in SDK's QuickSort

fpdfsdk/include/fsdk_mgr.h

 // 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
 
 #ifndef _FPDFSDK_MGR_H
 #define _FPDFSDK_MGR_H
 
 #include "fsdk_common.h"
@@ skipping 546 matching lines @@
                         
                         int iGreate = (*pCompare)(Value1, Value2);
                         if ((bAscend && iGreate > 0) || (!bAscend && iGreate < 0))
                         {
                                 this->SetAt(nStartPos, Value2);
                                 this->SetAt(nStopPos, Value1);
                         }
                         return;
                 }
                 
-                FX_UINT m = (nStartPos + nStopPos) / 2;
+                FX_UINT m = nStartPos + (nStopPos - nStartPos) / 2;
                 FX_UINT i = nStartPos;
                 
                 TYPE Value = this->GetAt(m);
                 
                 while (i < m)
                 {
                         TYPE temp = this->GetAt(i);
                         
                         int iGreate = (*pCompare)(temp, Value);
                         if ((bAscend && iGreate > 0) || (!bAscend && iGreate < 0))
@@ skipping 28 matching lines @@
                 }
                 
                 if (nStartPos < m) QuickSort(nStartPos, m, bAscend, pCompare);
                 if (nStopPos > m) QuickSort(m, nStopPos, bAscend, pCompare);
         }
 };
 
 
 #endif //_FPDFSDK_MGR_H