Fix segv in CPDF_DataAvail::CheckRoot() when /Root object is a string.

core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp

Index: core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
diff --git a/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp b/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
index 16274088c0de9b5d042d77824feb796be5f7c3cf..4ed4c70e27f581f9aae406c3911c89cd55379959 100644
--- a/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
+++ b/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
@@ -3390,7 +3390,12 @@ FX_BOOL CPDF_DataAvail::CheckRoot(IFX_DownloadHints* pHints)
         }
         return FALSE;
     }
-    CPDF_Reference* pRef = (CPDF_Reference*)m_pRoot->GetDict()->GetElement(FX_BSTRC("Pages"));
+    CPDF_Dictionary* pDict = m_pRoot->GetDict();
+    if (!pDict) {
+        m_docStatus = PDF_DATAAVAIL_ERROR;
+        return FALSE;
+    }
+    CPDF_Reference* pRef = (CPDF_Reference*)pDict->GetElement(FX_BSTRC("Pages"));
     if (pRef == NULL || pRef->GetType() != PDFOBJ_REFERENCE) {
         m_docStatus = PDF_DATAAVAIL_ERROR;
         return FALSE;