Insure width/height do not return a negative number.

src/core.js

@@ -798,7 +798,7 @@ jQuery.extend({
 			else
 				jQuery.swap( elem, props, getWH );
 
-			return val;
+			return Math.max(0, val);
 		}
 
 		return jQuery.curCSS( elem, name, force );