Fix a null object check

core/src/fpdfdoc/doc_basic.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/fpdfdoc/fpdf_doc.h"
 const int nMaxRecursion = 32;
 int CPDF_Dest::GetPageIndex(CPDF_Document* pDoc)
 {
@@ skipping 28 matching lines @@
         return pPage->GetObjNum();
     }
     return 0;
 }
 const FX_CHAR* g_sZoomModes[] = {"XYZ", "Fit", "FitH", "FitV", "FitR", "FitB", "FitBH", "FitBV", ""};
 int CPDF_Dest::GetZoomMode()
 {
     if (m_pObj == NULL || m_pObj->GetType() != PDFOBJ_ARRAY) {
         return 0;
     }
-    CFX_ByteString mode = ((CPDF_Array*)m_pObj)->GetElementValue(1)->GetString();
+    CFX_ByteString mode;
+    CPDF_Object* pObj = ((CPDF_Array*)m_pObj)->GetElementValue(1);
+    mode = pObj ? pObj->GetString() : CFX_ByteString();
     int i = 0;
     while (g_sZoomModes[i][0] != '\0') {
         if (mode == g_sZoomModes[i]) {
             return i + 1;
         }
         i ++;
     }
     return 0;
 }
 FX_FLOAT CPDF_Dest::GetParam(int index)
@@ skipping 501 matching lines @@
     if (nPage > 0 && nPage <= nPages) {
         return nPage;
     }
     return -1;
 }
 FX_INT32 CPDF_PageLabel::GetPageByLabel(FX_WSTR wsLabel) const
 {
     CFX_ByteString bsLabel = PDF_EncodeText((CFX_WideString)wsLabel);
     return GetPageByLabel(bsLabel);
 }