Fix a NULL pointer crash in the CJBig2_Image copy constructor.

core/src/fxcodec/jbig2/JBig2_Image.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 <limits.h>
 
 #include "JBig2_Image.h"
 #include "core/include/fxcrt/fx_coordinates.h"
@@ skipping 18 matching lines @@
 CJBig2_Image::CJBig2_Image(int32_t w,
                            int32_t h,
                            int32_t stride,
                            uint8_t* pBuf) {
   m_nWidth = w;
   m_nHeight = h;
   m_nStride = stride;
   m_pData = pBuf;
   m_bNeedFree = FALSE;
 }
-CJBig2_Image::CJBig2_Image(CJBig2_Image& im) {
+CJBig2_Image::CJBig2_Image(const CJBig2_Image& im) {
   m_nWidth = im.m_nWidth;
   m_nHeight = im.m_nHeight;
   m_nStride = im.m_nStride;
   if (im.m_pData) {
     m_pData = FX_Alloc2D(uint8_t, m_nStride, m_nHeight);
     JBIG2_memcpy(m_pData, im.m_pData, m_nStride * m_nHeight);
   } else {
     m_pData = NULL;
   }
   m_bNeedFree = TRUE;
@@ skipping 1040 matching lines @@
           dp[2] = (uint8_t)(tmp >> 8);
           dp[3] = (uint8_t)tmp;
         }
         lineSrc += m_nStride;
         lineDst += pDst->m_nStride;
       }
     }
   }
   return 1;
 }