Added line count check to avoid out of bounds

src/Markdig.Tests/TestStringLineGroup.cs

@@ -0,0 +1,55 @@
+using Markdig.Helpers;
+using System.Text;
+
+namespace Markdig.Tests;
+
+[TestFixture]
+public class TestStringLineGroup
+{
+    private static string ToString(ICharIterator text)
+    {
+        var chars = new StringBuilder();
+        while (text.CurrentChar != '\0')
+        {
+            chars.Append(text.CurrentChar);
+            text.NextChar();
+        }
+        return chars.ToString();
+    }
+
+    [Test]
+    public void TestStringLineGroupCharIteratorAtCapacity()
+    {
+        string str = "ABCDEFGHI";
+        var text = new StringLineGroup(1)
+        {
+            // Will store the following line at capacity
+            new StringSlice(str, NewLine.CarriageReturnLineFeed) { Start = 0, End = 2 },
+        };
+
+        var iterator = text.ToCharIterator();
+        var chars = ToString(iterator);
+        TextAssert.AreEqual("ABC\r\n", chars.ToString());
+        TextAssert.AreEqual("ABC", text.ToString());
+    }
+
+    [Test]
+    public void TestStringLineGroupCharIteratorForcingIncreaseCapacity()
+    {
+        string str = "ABCDEFGHI";
+        var text = new StringLineGroup(1)
+        {
+            // Will store the following line at capacity
+            new StringSlice(str, NewLine.CarriageReturnLineFeed) { Start = 0, End = 2 },
+
+            // Will force increase capacity to 2 and store the line at capacity
+            new StringSlice(str, NewLine.CarriageReturnLineFeed) { Start = 3, End = 3 },
+        };
+
+        var iterator = text.ToCharIterator();
+        var chars = ToString(iterator);
+        TextAssert.AreEqual("ABC\r\nD\r\n", chars.ToString());
+        TextAssert.AreEqual("ABC\r\nD", text.ToString());
+    }
+
+}

src/Markdig/Helpers/StringLineGroup.cs

@@ -334,9 +334,12 @@ public struct StringLineGroup : IEnumerable
             goto Return;
 
         MoveToNewLine:
-            SliceIndex++;
-            _offset = -1;
-            _currentSlice = _lines.Lines[SliceIndex];
+            if (SliceIndex < _lines.Count - 1)
+            {
+                SliceIndex++;
+                _offset = -1;
+                _currentSlice = _lines.Lines[SliceIndex];
+            }
 
         Return:
             return CurrentChar;