[bugfix] Fix edge case in Kramdown converter

weird combination of LI and P tags can cause the converter to crash

fixes gettalong#809

lib/kramdown/converter/kramdown.rb

@@ -146,7 +146,7 @@ def convert_li(el, opts)
         if el.children.first && el.children.first.type == :p && !el.children.first.options[:transparent]
           res = +"#{sym}#{text}"
           res << "^\n" if el.children.size == 1 && @stack.last.children.last == el &&
-            (@stack.last.children.any? {|c| c.children.first.type != :p } || @stack.last.children.size == 1)
+            (@stack.last.children.any? {|c| c.children.first && c.children.first.type != :p } || @stack.last.children.size == 1)
           res
         elsif el.children.first && el.children.first.type == :codeblock
           "#{sym}\n    #{text}"

test/test_converter_kramdown.rb

@@ -0,0 +1,37 @@
+# -*- coding: utf-8; frozen_string_literal: true -*-
+#
+#--
+# Copyright (C) 2009-2019 Thomas Leitner <[email protected]>
+#
+# This file is part of kramdown which is licensed under the MIT.
+#++
+#
+
+require 'minitest/autorun'
+require 'kramdown/converter/kramdown'
+
+describe Kramdown::Converter::Kramdown do
+  it "converts weird html to kramdown" do
+    html = <<~HTML
+<ul>
+  <li></li>
+  <li>
+    <p>one</p>
+  </li>
+  <li>
+    <p>two</p>
+  </li>
+</ul>
+    HTML
+    doc, = Kramdown::Parser::Html.parse(html)
+    md, = Kramdown::Converter::Kramdown.convert(doc)
+    expected = <<~MD
+* 
+* one
+
+* two
+
+    MD
+    assert_equal expected, md
+  end
+end