[pdfmod] [PdfSharp] Fix crash if Catalog.Outlines is PdfNull (bgo#633123)

[Gabriel Burt <gburt src gnome org>]

commit 7e1ef2f2513bf31dcb613ce97b066595f00a1827
Author: Gabriel Burt <gabriel burt gmail com>
Date:   Thu Mar 3 12:15:37 2011 -0600

    [PdfSharp] Fix crash if Catalog.Outlines is PdfNull (bgo#633123)

 lib/PdfSharp/PdfSharp.Pdf.Advanced/PdfCatalog.cs |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/lib/PdfSharp/PdfSharp.Pdf.Advanced/PdfCatalog.cs b/lib/PdfSharp/PdfSharp.Pdf.Advanced/PdfCatalog.cs
index f785f00..b55420b 100644
--- a/lib/PdfSharp/PdfSharp.Pdf.Advanced/PdfCatalog.cs
+++ b/lib/PdfSharp/PdfSharp.Pdf.Advanced/PdfCatalog.cs
@@ -147,10 +147,20 @@ namespace PdfSharp.Pdf.Advanced
       {
         if (this.outline == null) {
           try {
-            this.outline = (PdfOutline)Elements.GetValue(Keys.Outlines, VCF.CreateIndirect);
+            var obj = Elements.GetValue(Keys.Outlines, VCF.CreateIndirect);
+            this.outline = obj as PdfOutline;
+
+            // If there's a PdfNull object there (pdfopt does this) then remove it
+            // and replace it with a real PdfOutline so that the user can edit the outlines/bookmarks
+            // if they want to
+            if (this.outline == null && obj is PdfNull) {
+              Elements.Remove (Keys.Outlines);
+              this.outline = Elements.GetValue(Keys.Outlines, VCF.CreateIndirect) as PdfOutline;
+            }
           } catch (InvalidOperationException) {
+            // This happens if the Outline iref points to a null value; remove it and start over
             Elements.Remove (Keys.Outlines);
-            this.outline = (PdfOutline)Elements.GetValue(Keys.Outlines, VCF.CreateIndirect);
+            this.outline = Elements.GetValue(Keys.Outlines, VCF.CreateIndirect) as PdfOutline;
           }
         }
         return this.outline.Outlines;