FIX : undefined references is a valid use case.

I tried to mitigate the breaking change by keep on throwing in most uses of the change method.

src/UglyToad.PdfPig.Tests/Tokenization/Scanner/PdfTokenScannerTests.cs

@@ -50,6 +50,36 @@ endobj";
             var reference = Assert.IsType<IndirectReferenceToken>(token.Data);
 
             Assert.Equal(new IndirectReference(12, 7), reference.Data);
+        }
+
+        [Fact]
+        public void ReadsObjectWithUndefinedIndirectReference()
+        {
+            const string s = @"
+5 0 obj
+<<
+/XObject <<
+/Pic1 7 0 R
+>>
+/ProcSet [/PDF /Text /ImageC ]
+/Font <<
+/F0 8 0 R
+/F1 9 0 R
+/F2 10 0 R
+/F3 0 0 R
+>>
+>>
+endobj";
+
+            var scanner = GetScanner(s);
+
+            ReadToEnd(scanner);
+
+            var token = scanner.Get(new IndirectReference(5, 0));
+            Assert.NotNull(token);
+
+            token = scanner.Get(new IndirectReference(0, 0));
+            Assert.Null(token);
         }
 
         [Fact]