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/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>