Markdown.ToHtml throws IndexOutOfRangeException for lastBlock in GetCurrentDefinitionList

[mschessler]

Tested Version: newest commit d1233ff on main

I did not really take a closer look why this is happening but the problem is DefinitionListParser.GetCurrentDefinitionList() calling previousParent with index = -1 if the lastBlock is a BlankLineBlock.

if (lastBlock is BlankLineBlock)
{
    lastBlock = previousParent[index - 1];
    previousParent.RemoveAt(index);
}

Casting -1 to an unsigned int is then problematic.

Reproducer:

var pipeline = new MarkdownPipelineBuilder().UseAdvancedExtensions().Build();
var result = Markdown.ToHtml("+\r\n\r\n+\r\n\r\n\t\f\r\n~\t", pipeline);

or

var pipeline = new MarkdownPipelineBuilder().UseAdvancedExtensions().Build();
var result = Markdown.ToHtml("- \r\n\r\n\t\f\r\n~\t", pipeline);

Stacktrace:

ThrowHelper removed for Stacktrace.

at Markdig.Syntax.ContainerBlock.get_Item(Int32 index) in \src\Markdig\Syntax\ContainerBlock.cs:line 220
at Markdig.Extensions.DefinitionLists.DefinitionListParser.GetCurrentDefinitionList(ParagraphBlock paragraphBlock, ContainerBlock previousParent) in \src\Markdig\Extensions\DefinitionLists\DefinitionListParser.cs:line 111
at Markdig.Extensions.DefinitionLists.DefinitionListParser.TryOpen(BlockProcessor processor) in \src\Markdig\Extensions\DefinitionLists\DefinitionListParser.cs:line 53
at Markdig.Parsers.BlockProcessor.TryOpenBlocks(BlockParser[] parsers) in \src\Markdig\Parsers\BlockProcessor.cs:line 821
at Markdig.Parsers.BlockProcessor.TryOpenBlocks() in \src\Markdig\Parsers\BlockProcessor.cs:line 763
at Markdig.Parsers.BlockProcessor.ProcessLine(StringSlice newLine) in \src\Markdig\Parsers\BlockProcessor.cs:line 501
at Markdig.Parsers.MarkdownParser.ProcessBlocks(BlockProcessor blockProcessor, String text) in \src\Markdig\Parsers\MarkdownParser.cs:line 113
at Markdig.Parsers.MarkdownParser.Parse(String text, MarkdownPipeline pipeline, MarkdownParserContext context) in \src\Markdig\Parsers\MarkdownParser.cs:line 56
at Markdig.Markdown.ToHtml(String markdown, MarkdownPipeline pipeline, MarkdownParserContext context) in \src\Markdig\Markdown.cs:line 101
at FuzzTest.Program.Main(String[] args) in \FuzzingTargets\MarkdigFuzzer\Program.cs:line 14