Add some nullptr checks that were missing from commit f7992b39.

core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp

Index: core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp
diff --git a/core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp b/core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp
index 187d6152e96ceccc76c7f7ee4be604d6aa1d40da..9adfdb21a4c92b744435f19494a960d1983a2ef7 100644
--- a/core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp
+++ b/core/src/fpdfapi/fpdf_page/fpdf_page_parser_old.cpp
@@ -92,7 +92,7 @@ void CPDF_StreamContentParser::Handle_BeginImage() {
     nonstd::unique_ptr<CPDF_Object, ReleaseDeleter<CPDF_Object>> pObj(
         m_pSyntax->ReadNextObject());
     if (!key.IsEmpty()) {
-      FX_DWORD dwObjNum = pObj->GetObjNum();
+      FX_DWORD dwObjNum = pObj ? pObj->GetObjNum() : 0;
       if (dwObjNum)
         pDict->SetAtReference(key, m_pDocument, dwObjNum);
       else