fix resolving compressed objects in additional cross-reference table

[delta-emil]

It looks like there is a problem with resolving compressed objects (Type==2) in additional cross-reference table (referenced by /Prev in the main one), when the main one also references it in its own stream as a Type==1 object.

The problem seems to be that the additional cross-reference table is getting added to the xrefTable with no Vaule, just position, during the precessing of the main cross-reference table object when it's a Type==1 reference.

Then when the code uses the /Prev to also processes the additional cross-reference table, it tries to add it using xrefTable.Add(iref), but that does nothing because its objectID is already in the xrefTable (with the position, and null Value).

This then hinders the mechanisms in PdfReader:

1. PdfReader needs to find a PdfCrossReferenceStream in iref.Value in order to resolve compressed objects. This causes the objects to be missing later.
2. PdfReader, would also in the end, seeing null in iref.Value, parse the additional cross-reference object anew redundantly.

Unfortunately, I could not create a test case that I can share, because the file that revealed the bug contains data I am not allowed to share; and I'm not proficient enough with the pdf format to craft a sample pdf file from scratch.

PS: I saw that people reported an exception that look exactly as the one I was getting as issue #432, and it might be causing other hard-to-find problems if because of the missing objects.