Add destroy() to Control.OverviewMap and add a check to map.destroy() to bail if it's already been called (by, say, window.onunload). Fixes #595.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@2958 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Map.js

@@ -237,6 +237,10 @@ OpenLayers.Map.prototype = {
     * @private
     */
     destroy:function() {
+        // if unloadDestroy is null, we've already been destroyed
+        if (!this.unloadDestroy) {
+            return false;
+        }
 
         // map has been destroyed. dont do it again!
         OpenLayers.Event.stopObserving(window, 'unload', this.unloadDestroy);