patch for #509 -- fixes script error when tile is destroyed before it is used. thx cr5

git-svn-id: http://svn.openlayers.org/trunk/openlayers@2286 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Util.js

@@ -232,7 +232,7 @@ OpenLayers.Util.onImageLoad = function() {
     // this code that turns its display on).
     //
     if (!this.viewRequestID ||
-        (this.viewRequestID == this.map.viewRequestID)) { 
+        (this.map && this.viewRequestID == this.map.viewRequestID)) { 
         this.style.backgroundColor = null;
         this.style.display = "";  
     }
@@ -954,4 +954,4 @@ OpenLayers.Util.removeTail = function(url) {
                                   : url.substr(0, qMark);
     }
     return head;
-};
+};