Odd indented code block can break paragraph

[dbuenzli]

and the spec says indented code blocks don't interrupt paragraphs

> md2html --version
0.4.8

> printf 'a\n    ```\nb' | md2html
<p>a</p>
<pre><code>b
</code></pre>

> printf 'a\n    ```\nb' | cmark  
<p>a
```
b</p>

Likely something fishy happening because the indented text looks like a fence: for example md2html passes this spec test, and it is correct if you only have two backticks or less.

[mity]

Same as #201, should be fixed.