Treat null Parent as no Parent for dictionary objects

To be complient with PDF reference chapter 7.3.7

BUG=402437
R=vitalybuka@chromium.org

Review URL: https://codereview.chromium.org/469573002

fpdfsdk/src/fpdfppo.cpp

diff --git a/fpdfsdk/src/fpdfppo.cpp b/fpdfsdk/src/fpdfppo.cpp
index 032c3db..b8d2125 100644
--- a/fpdfsdk/src/fpdfppo.cpp
+++ b/fpdfsdk/src/fpdfppo.cpp
@@ -215,8 +215,11 @@
 	{
 		if(pp->KeyExist((const char*)nSrctag))	
 			return pp->GetElement((const char*)nSrctag);
-		else if(pp->KeyExist("Parent"))
+		else if (pp->KeyExist("Parent"))
+		{
 			pp = (CPDF_Dictionary*)pp->GetElement("Parent")->GetDirect();
+			if (pp->GetType() == PDFOBJ_NULL) break;
+		}
 		else break;
 	}