Fix regression in JBIG2 decoding from commit ce37d73.

core/src/fxcodec/jbig2/JBig2_ArithIntDecoder.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 CORE_SRC_FXCODEC_JBIG2_JBIG2_ARITHINTDECODER_H_
 #define CORE_SRC_FXCODEC_JBIG2_JBIG2_ARITHINTDECODER_H_
 
 #include <vector>
 
 #include "../../../include/fxcrt/fx_system.h"
 #include "JBig2_ArithDecoder.h"
 
 class CJBig2_ArithIntDecoder {
  public:
   CJBig2_ArithIntDecoder();
   ~CJBig2_ArithIntDecoder();
 
+  // Returns true on success, and false when an OOB condition occurs. Many
+  // callers can tolerate OOB and do not check the return value.
   bool decode(CJBig2_ArithDecoder* pArithDecoder, int* nResult);
 
  private:
   std::vector<JBig2ArithCtx> m_IAx;
 };
 
 class CJBig2_ArithIaidDecoder {
  public:
   explicit CJBig2_ArithIaidDecoder(unsigned char SBSYMCODELENA);
   ~CJBig2_ArithIaidDecoder();
 
   void decode(CJBig2_ArithDecoder* pArithDecoder, FX_DWORD* nResult);
 
  private:
   std::vector<JBig2ArithCtx> m_IAID;
 
   const unsigned char SBSYMCODELEN;
 };
 
 #endif  // CORE_SRC_FXCODEC_JBIG2_JBIG2_ARITHINTDECODER_H_