Fix a heap overflow in CJBig2_Context::parseSymbolDict

core/src/fxcodec/jbig2/JBig2_ArithDecoder.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 _JBIG2_ARITHMETIC_DECODER_H_
 #define _JBIG2_ARITHMETIC_DECODER_H_
 #include "JBig2_Define.h"
 #include "JBig2_BitStream.h"
@@ skipping 54 matching lines @@
         }
     } else {
         m_pStream->incByteIdx();
         B = m_pStream->getCurByte_arith();
         C = C + 0xff00 - (B << 8);
         CT = 8;
     }
 }
 inline int CJBig2_ArithDecoder::DECODE(JBig2ArithCtx *pCX)
 {
+    if (!pCX || pCX->I >= JBIG2_QE_NUM) {
+        return 0;
+    }
+
     int D;
     const JBig2ArithQe * qe = &QeTable[pCX->I];
     A = A - qe->Qe;
     if((C >> 16) < A) {
         if(A & 0x8000) {
             D = pCX->MPS;
         } else {
             if(A < qe->Qe) {
                 D = 1 - pCX->MPS;
                 if(qe->nSwitch == 1) {
@@ skipping 32 matching lines @@
                 BYTEIN();
             }
             A <<= 1;
             C <<= 1;
             CT--;
         } while ((A & 0x8000) == 0);
     }
     return D;
 }
 #endif