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/drawer.go

@@ -67,6 +67,9 @@ func (d *Document) parsePropertyDrawer(i int, parentStop stopFn) (int, Node) {
 	}
 	for ; !stop(d, i); i++ {
 		m := propertyRegexp.FindStringSubmatch(d.tokens[i].matches[0])
+		if m == nil {
+			return 0, nil
+		}
 		k, v := strings.ToUpper(m[2]), strings.TrimSpace(m[4])
 		drawer.Properties = append(drawer.Properties, []string{k, v})
 	}

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>

org/testdata/headlines.org

@@ -27,3 +27,7 @@ it's possible to use =#+SETUPFILE= - in this case the setup file contains the fo
 this headline and it's content are not exported as it is marked with an =EXCLUDE_TAGS= tag.
 By default =EXCLUDE_TAGS= is just =:noexport:=.
 
+* malformed property drawer
+:PROPERTIES:
+not a property
+:END:

org/testdata/headlines.pretty_org

@@ -27,3 +27,7 @@ it's possible to use =#+SETUPFILE= - in this case the setup file contains the fo
 this headline and it's content are not exported as it is marked with an =EXCLUDE_TAGS= tag.
 By default =EXCLUDE_TAGS= is just =:noexport:=.
 
+* malformed property drawer
+:PROPERTIES:
+not a property
+:END:

org/testdata/misc.html

@@ -145,6 +145,11 @@ it's possible to use =#+SETUPFILE= - in this case the setup file contains th
 * excluded headline                                                :custom_noexport:
 this headline and it's content are not exported as it is marked with an =EXCLUDE_TAGS= tag.
 By default =EXCLUDE_TAGS= is just =:noexport:=.
+
+* malformed property drawer
+:PROPERTIES:
+not a property
+:END:
 </pre>
 </div>
 </div>