Fix #45, errors when ul/ol allowed without li.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

library/HTMLPurifier/ChildDef/List.php

@@ -38,6 +38,12 @@ class HTMLPurifier_ChildDef_List extends HTMLPurifier_ChildDef
             return false;
         }
 
+        // if li is not allowed, delete parent node
+        if (!isset($config->getHTMLDefinition()->info['li'])) {
+            trigger_error("Cannot allow ul/ol without allowing li", E_USER_WARNING);
+            return false;
+        }
+
         // the new set of children
         $result = array();