Colons in footnotes generate links w/o `href`

[wooorm]

[^:]

[^:]: a

Yields:

<p dir="auto">
  <sup>
    <a id="user-content-fnref-:" data-footnote-ref="" aria-describedby="footnote-label">1</a>
  </sup>
</p>
<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
<ol dir="auto">
<li id="user-content-fn-:">
<p dir="auto">a ↩</p>
</li>
</ol>
</section></div>

Yields:

^:

---

Expected:

<a href="#user-content-fn-:" and a normal backlink in the list.

---

Related to https://github.com/github/cmark-gfm/issues/241