update oldDisplayName value on displayName change

This patch fixes a bug that can be reproduced in following way:

1. change displayName
2. clear text in displayName input click change again

Then you will get an error and previous displayName will be restored.

Expected behavior should restore the new updated displayName.

settings/js/personal.js

@@ -42,6 +42,7 @@ $(document).ready(function(){
 			$.post( 'ajax/changedisplayname.php', post, function(data){
 				if( data.status == "success" ){
 					$('#displaynamechanged').show();
+					$('#oldDisplayName').val($('#displayName').val());
 					// update displayName on the top right expand button
 					str_parts = $('#expand').html().split('\t');
 					str_parts[5] = $('#displayName').val();