Fixing IFrame tile layers.

A problem introduced with 5fda8835da can easily be solved by re-adding a check for imgDiv and a strict type check for useIFrame. Thanks @elemoine for pointing out that there could be a problem.

lib/OpenLayers/Tile/Image.js

@@ -339,6 +339,9 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
      * or if it's currently loading.
      */
     createBackBuffer: function() {
+        if (!this.imgDiv || this.isLoading) {
+            return;
+        }
         var backBuffer;
         if (this.frame) {
             backBuffer = this.frame.cloneNode(false);