Avoid a PHP notice in bundled themes if no images were found.

props nabil_kadimi.
fixes #28361.
Built from https://develop.svn.wordpress.org/trunk@28585


git-svn-id: http://core.svn.wordpress.org/trunk@28410 1a063a9b-81f0-0310-95a4-ce76da25c4cd

image.php

@@ -58,9 +58,9 @@ foreach ( $attachments as $k => $attachment ) :
 		break;
 endforeach;
 
-$k++;
 // If there is more than 1 attachment in a gallery
 if ( count( $attachments ) > 1 ) :
+	$k++;
 	if ( isset( $attachments[ $k ] ) ) :
 		// get the URL of the next image attachment
 		$next_attachment_url = get_attachment_link( $attachments[ $k ]->ID );