add another edge-case to the code that reads xref tables

add test

resolves DEVSIX-1426

Autoported commit.
Original commit hash: [1b98844b04]

Author: joris.schellekens

itext.tests/itext.kernel.tests/itext/kernel/pdf/CorruptXRefTest.cs

@@ -0,0 +1,43 @@
+using System;
+using iText.Kernel.Utils;
+using iText.Test;
+
+namespace iText.Kernel.Pdf {
+    /// <summary>
+    /// This test checks correct handling of pdf documents with (slightly) corrupt XREF table
+    /// xref
+    /// 0 30
+    /// 0000000000 65535 f
+    /// 0000000000 65535 f
+    /// </summary>
+    public class CorruptXRefTest {
+        public static readonly String sourceFolder = iText.Test.TestUtil.GetParentProjectDirectory(NUnit.Framework.TestContext
+            .CurrentContext.TestDirectory) + "/resources/itext/kernel/pdf/CorruptXRefTest/";
+
+        public static readonly String outputFolder = NUnit.Framework.TestContext.CurrentContext.TestDirectory + "/test/itext/kernel/pdf/CorruptXRefTest/";
+
+        [NUnit.Framework.OneTimeSetUp]
+        public static void BeforeClass() {
+            ITextTest.CreateDestinationFolder(outputFolder);
+        }
+
+        /// <exception cref="System.IO.IOException"/>
+        /// <exception cref="System.Exception"/>
+        [NUnit.Framework.Test]
+        public virtual void ReadPdfWithCorruptXRef() {
+            String cmpFile = sourceFolder + "cmp_docOut.pdf";
+            String outputFile = outputFolder + "docOut.pdf";
+            String inputFile = sourceFolder + "docIn.pdf";
+            PdfWriter writer = new PdfWriter(outputFile);
+            PdfReader reader = new PdfReader(inputFile);
+            PdfDocument inputPdfDocument = new PdfDocument(reader);
+            PdfDocument outputPdfDocument = new PdfDocument(writer);
+            int lastPage = inputPdfDocument.GetNumberOfPages();
+            inputPdfDocument.CopyPagesTo(lastPage, lastPage, outputPdfDocument);
+            inputPdfDocument.Close();
+            outputPdfDocument.Close();
+            NUnit.Framework.Assert.IsNull(new CompareTool().CompareByContent(outputFolder + "docOut.pdf", cmpFile, outputFolder
+                , "diff_"));
+        }
+    }
+}

itext.tests/itext.kernel.tests/resources/itext/kernel/pdf/CorruptXRefTest/cmp_docOut.pdf

@@ -891,7 +891,7 @@ protected internal virtual PdfDictionary ReadXrefSection() {
                     tokens.NextValidToken();
                     int gen = tokens.GetIntValue();
                     tokens.NextValidToken();
-                    if (pos == 0L && gen == 65535 && num == 1) {
+                    if (pos == 0L && gen == 65535 && num == 1 && start != 0) {
                         // Very rarely can an XREF have an incorrect start number. (SUP-1557)
                         // e.g.
                         // xref

itext.tests/itext.kernel.tests/resources/itext/kernel/pdf/CorruptXRefTest/docIn.pdf

@@ -1 +1 @@
-28bdf7fff3502700d5093408fa91d35c180e5973
+1b98844b04ed0e75883e0a70706d8f36bbbf932d