fix stackoverflow on pdfmerger (#252)

Obj Ref1
 Kids: [Ref2]

Obj Ref2
 Parent: Ref1

src/UglyToad.PdfPig.Tests/Writer/PdfMergerTests.cs

@@ -2,6 +2,7 @@
 {
     using Integration;
     using PdfPig.Writer;
+    using System;
     using System.IO;
     using Xunit;
 
@@ -172,5 +173,22 @@
                 // ignored.
             }
         }
+
+        [Fact]
+        public void NoStackoverflow()
+        {
+            try
+            {
+                var bytes = PdfMerger.Merge(IntegrationHelpers.GetDocumentPath("68-1990-01_A.pdf"));
+                using (var document = PdfDocument.Open(bytes, ParsingOptions.LenientParsingOff))
+                {
+                    Assert.Equal(45, document.NumberOfPages);
+                }
+            }
+            catch (StackOverflowException)
+            {
+                Assert.True(false);
+            }
+        }
     }
 }

src/UglyToad.PdfPig/Writer/PdfMerger.cs

@@ -415,15 +415,15 @@
                                 return newReferenceToken;
                             }
 
-                            var tokenObject = DirectObjectFinder.Get<IToken>(referenceToken.Data, tokenScanner);
+                            //we add the token to referencesFromDocument to prevent stackoverflow on references cycles 
+                            newReferenceToken = context.ReserveNumberToken();
+                            referencesFromDocument.Add(referenceToken, newReferenceToken);
 
+                            var tokenObject = DirectObjectFinder.Get<IToken>(referenceToken.Data, tokenScanner);
                             Debug.Assert(!(tokenObject is IndirectReferenceToken));
 
                             var newToken = CopyToken(tokenObject, tokenScanner, referencesFromDocument);
-                            newReferenceToken = context.WriteToken(newToken);
+                            context.WriteToken(newReferenceToken, newToken);
-
-                            referencesFromDocument.Add(referenceToken, newReferenceToken);
-
                             return newReferenceToken;
                         }
                     case StreamToken streamToken:

src/UglyToad.PdfPig/Writer/PdfStreamWriter.cs

@@ -89,7 +89,11 @@
 
             // When we end up writing this token, all of his child would already have been added and checked for duplicate
             return AddToken(token, reservedNumber.Value);
+        }
 
+        public void WriteToken(IndirectReferenceToken referenceToken, IToken token)
+        {
+            tokenReferences.Add(referenceToken, token);
         }
 
         public int ReserveNumber()