Error when Markdown document contains two successive footnote markers

[peteroupc]

Kramdown version 2.5.1 raises an error in HTML conversion when a Markdown document contains two footnote markers (such as [^1]:) one after the other.

The following code demonstrates this issue.

require 'kramdown'

Kramdown::Document.new(<<__END__

A[^1] B[^2]

[^1]: [^2]: Note text 2

[^3]: Note text 3

__END__
).to_html

/usr/local/share/gems/gems/kramdown-2.5.1/lib/kramdown/converter/html.rb:507:in 'Kramdown::Converter::Html#footnote_content': undefined method 'type' for nil (NoMethodError)

          if li.children.last.type == :p || @options[:footnote_backlink_inline]
                             ^^^^^
        from /usr/local/share/gems/gems/kramdown-2.5.1/lib/kramdown/converter/html.rb:389:in 'Kramdown::Converter::Html#convert_root'
        from /usr/local/share/gems/gems/kramdown-2.5.1/lib/kramdown/converter/html.rb:58:in 'Kramdown::Converter::Html#convert'
        from /usr/local/share/gems/gems/kramdown-2.5.1/lib/kramdown/converter/base.rb:107:in 'Kramdown::Converter::Base.convert'
        from /usr/local/share/gems/gems/kramdown-2.5.1/lib/kramdown/document.rb:116:in 'Kramdown::Document#method_missing'
        from (irb):28:in '<main>'
        from <internal:kernel>:168:in 'Kernel#loop'
        from /usr/share/gems/gems/irb-1.14.3/exe/irb:9:in '<top (required)>'
        from /usr/bin/irb:25:in 'Kernel#load'
        from /usr/bin/irb:25:in '<main>'

By contrast, no error is raised in the following code:

require 'kramdown'

Kramdown::Document.new(<<__END__

A[^1] B[^2]

[^1]:

[^2]: Note text 2

[^3]: Note text 3

__END__
).to_html