Fixed several destroy() methods to avoid running into undefined objects when the page unloads. r=crschmidt,me (closes #1222)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@6172 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Control.js

@@ -166,6 +166,7 @@ OpenLayers.Control = OpenLayers.Class({
      */
     destroy: function () {
         this.events.destroy();
+        this.events = null;
         // eliminate circular references
         if (this.handler) {
             this.handler.destroy();