fix a few linux compilation warnings

core/src/fpdfapi/fpdf_render/fpdf_render_loadimage.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 "../../../include/fxge/fx_ge.h"
 #include "../../../include/fxcodec/fx_codec.h"
 #include "../../../include/fpdfapi/fpdf_module.h"
 #include "../../../include/fpdfapi/fpdf_render.h"
@@ skipping 313 matching lines @@
     src_pitch /= 8;
     src_pitch *= m_Height;
     if (!src_pitch.IsValid()) {
         return 0;
     }
     m_pStreamAcc = FX_NEW CPDF_StreamAcc;
     m_pStreamAcc->LoadAllData(pStream, FALSE, src_pitch.ValueOrDie(), TRUE);
     if (m_pStreamAcc->GetSize() == 0 || m_pStreamAcc->GetData() == NULL) {
         return 0;
     }
-    const CFX_ByteString& decoder = m_pStreamAcc->GetImageDecoder();
     int ret = CreateDecoder();
     if (ret != 1) {
         if (!ret) {
             return ret;
         }
         if (!ContinueToLoadMask()) {
             return 0;
         }
         if (m_bHasMask) {
             StratLoadMask();
@@ skipping 1222 matching lines @@
     if (!m_bCached) {
         if (m_pBitmap) {
             delete m_pBitmap;
             m_pBitmap = NULL;
         }
         if (m_pMask) {
             delete m_pMask;
         }
     }
 }