Add support for ObjectStream in PDF

ObjectStream isn't necessarely malicious, but can be. This patch could
be improved by unpacking the content of the stream, but it requires 3rd
party libraries we don't have for now.

Final fix for PCL-01-002

bin/filecheck.py

@@ -407,6 +407,8 @@ class File(FileBase):
             self.make_dangerous('Pdf with launch action(s)')
         if oPDFiD.xfa.count > 0:
             self.make_dangerous('Pdf with XFA structures')
+        if oPDFiD.objstm.count > 0:
+            self.make_dangerous('Pdf with ObjectStream structures')
         if not self.is_dangerous:
             self.add_description('Pdf file')