message: fix mam button in empty windows
diff: @@ -166,7 +166,6 @@ jsxc.xmpp.mam.initWindow = function(ev, win) {
  }
  var bid = win.attr('data-bid');
  - var buddyData = jsxc.storage.getUserItem('buddy', bid) || {};
  var element = $('<div>');
  element.addClass('jsxc_mam-load-more');
  @@ -177,6 +176,8 @@ jsxc.xmpp.mam.initWindow = function(ev, win) {
  element.text($.t('Load_older_messages'));
  win.find('.jsxc_textarea').scroll(function() {
  + var buddyData = jsxc.storage.getUserItem('buddy', bid) || {};
  +
  if (this.scrollTop < 42 && !buddyData.archiveExhausted) {
  element.addClass('jsxc_show');
  } else {
  