Fix a null object check

BUG=390615
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/378293005

core/src/fpdfdoc/doc_basic.cpp

diff --git a/core/src/fpdfdoc/doc_basic.cpp b/core/src/fpdfdoc/doc_basic.cpp
index 25f641c..5ccf4ce 100644
--- a/core/src/fpdfdoc/doc_basic.cpp
+++ b/core/src/fpdfdoc/doc_basic.cpp
@@ -46,7 +46,9 @@
     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]) {