Bad definition list parsing

[jgm]

% commonmark -xdefinition_lists
## Blah

`-v`, `--version`

:   Print version.

`-h`, `--help`

:   Show usage message.
^D
<h2 id="blah">Blah</h2>
<dl>
<dt><code>-v</code>, <code>--version</code></dt>
<dd>
<p>Print version.</p>
</dd>
</dl>
<dl>
<dt><code>-h</code>, <code>--help</code></dt>
<dd>
<p>Show usage message.</p>
</dd>
</dl>

This produces two dls rather than one. But if you remove the heading, you get one as intended.

[jgm]

Actually it doesn't matter if the first line is a heading.
Could be anything.

[jgm]

Doesn't matter whether the def list is tight or loose.

[jgm]

Minimal repro:

x

a
:    b

c
:    d