OldStylePdfWriter writes wrong /Size in trailer, XRef doesn't count objects correctly

[Gwurk <gw...@fi...>]

Hi again,

I noticed a problem where PDFedit seems to write an incorrect /Size
entry in the new trailer it adds. Suppose the PDF we're editing has 5
entries in its xref table, but only 2 are used:

xref
0 5
0000000001 65535 f
0000000002 00001 f
0000000000 00001 f
0000000123 00000 n
0000000456 00000 n
trailer << /Size 5  /Root 4 0 R >>

In OldStylePdfWriter::writeTrailer(), we compute the new /Size as:
    std::max(prevSection.objNum, (size_t)(maxObjNum + 1))

and prevSection.objNum is set by the caller XRefWriter::saveChanges():
    secInfo={lastXRefPos, XRef::getNumObjects()};

And looking at XRef::getNumObjects(), it's defined as:
    virtual int getNumObjects()
    {
       int count=0;
       for(int i=0; i<size; i++)
          // counts just not free entries
          if(entries[i].type != xrefEntryFree)
             count++;

       return count;
    }

Notice that it just counts the number of used entries; it does not
return the number of the LAST used entry. So XRef::getNumObjects()
would return 2, even though the xref table had 5 entries. If we wrote
a new trailer with this information, it would have the wrong /Size.