Applied bartvde's patch for #498.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@2225 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Control/OverviewMap.js

@@ -106,6 +106,7 @@ OpenLayers.Control.OverviewMap.prototype =
         this.extentRectangle = document.createElement('div');
         this.extentRectangle.style.position = 'absolute';
         this.extentRectangle.style.zIndex = 1000;  //HACK
+        this.extentRectangle.style.overflow = 'hidden';
         this.extentRectangle.style.backgroundImage = 'url(' +
                                         OpenLayers.Util.getImagesLocation() +
                                         '/blank.png)';