A fix for #440. Prevent people who somehow get a null element in the event

cache from shooting themselves in the foot: This doesn't seem to happen often,
but it has happened in internal code. I'm going to add more debugging info to 
#440 as we get it, but want to get this in to prevent people from shooting 
themselves in the foot in IE accidentally.


git-svn-id: http://svn.openlayers.org/trunk/openlayers@2024 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Events.js

@@ -96,9 +96,9 @@ OpenLayers.Event = {
         || element.detachEvent))
       name = 'keydown';
 
-    if (element.removeEventListener) {
+    if (element && element.removeEventListener) {
       element.removeEventListener(name, observer, useCapture);
-    } else if (element.detachEvent) {
+    } else if (element && element.detachEvent) {
       element.detachEvent('on' + name, observer);
     }
   }