Some plain/text line cause System.IndexOutOfRangeException

[msa48]

test case
some line ends with UrlScanner pattern ftp.

this couse to exception

   в MimeKit.Text.UrlScanner.SkipSubDomain(Char[] text, Int32 endIndex, Int32& index)
   в MimeKit.Text.UrlScanner.SkipDomain(Char[] text, Int32 endIndex, Int32& index)
   в MimeKit.Text.UrlScanner.GetWebEndIndex(UrlMatch match, Char[] text, Int32 startIndex, Int32 matchIndex, Int32 endIndex)
   в MimeKit.Text.UrlScanner.Scan(Char[] text, Int32 startIndex, Int32 count, UrlMatch& match)
   в MimeKit.Text.TextToHtml.WriteText(HtmlWriter htmlWriter, String text)
   в MimeKit.Text.TextToHtml.Convert(TextReader reader, TextWriter writer)
   в MimeKit.Text.TextConverter.Convert(String text)

because in this case endIndex is equal length of string