CommonMark rendering does not handle one case of nested emphasis

[egrimley]

$ printf '_a*b*c_' | cmark -t html   
<p><em>a<em>b</em>c</em></p>
$ printf '_a*b*c_' | cmark -t commonmark
*a*b*c*
$ printf '_a*b*c_' | cmark -t commonmark | cmark -t html
<p><em>a</em>b<em>c</em></p>