Themes: Ensure the image returns a width before checking that it is less than 300px.

see #26459.


git-svn-id: https://develop.svn.wordpress.org/trunk@26765 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/theme.js

@@ -366,7 +366,7 @@ themes.view.Details = wp.Backbone.View.extend({
 		image.src = screenshot.attr( 'src' );
 
 		// Width check
-		if ( image.width <= 300 ) {
+		if ( image.width && image.width <= 300 ) {
 		 	el.addClass( 'small-screenshot' );
 		}
 	}