Add some nullptr checks that were missing from commit f7992b39.

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 4393bfbfe8a324cb870e51e24aad8f09ed8b8b36..f6c0b1d72bf3b02f03b0a8d8d11087d9d677245c 100644
--- a/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
+++ b/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
@@ -852,7 +852,8 @@ FX_BOOL CPDF_Parser::RebuildCrossRef() {
                           CFX_ByteString key;
                           CPDF_Object* pElement =
                               pTrailer->GetNextElement(pos, key);
-                          FX_DWORD dwObjNum = pElement->GetObjNum();
+                          FX_DWORD dwObjNum =
+                              pElement ? pElement->GetObjNum() : 0;
                           if (dwObjNum) {
                             m_pTrailer->SetAtReference(key, m_pDocument,
                                                        dwObjNum);