Handle malformed :PROPERTY: drawers

Until now we'd get index out of range errors trying to index into the match if
:PROPERTIES: was followed by a non-property. Thanks fuzz

org/testdata/headlines.html

@@ -12,6 +12,8 @@
 </li>
 <li><a href="#headline-6">excluded headline</a>
 </li>
+<li><a href="#headline-7">malformed property drawer</a>
+</li>
 </ul>
 </nav>
 <h1 id="headline-1">
@@ -76,3 +78,13 @@ it&#39;s possible to use <code class="verbatim">#+SETUPFILE</code> - in this cas
 </pre>
 </div>
 </div>
+<h1 id="headline-7">
+malformed property drawer
+</h1>
+<p>
+:PROPERTIES:
+not a property
+</p>
+<p>
+:END:
+</p>