Fix IE JS Error if first item is disabled

If the first item is disabled, it cannot be focused. This raises a
JS error in IE8 (and probably other versions).

Make sure we find the first item that is not disabled and focus this.

Author: Markus Doits

src/jquery.multiselect.js

@@ -574,7 +574,7 @@
       // select the first option
       // triggering both mouseover and mouseover because 1.4.2+ has a bug where triggering mouseover
       // will actually trigger mouseenter.  the mouseenter trigger is there for when it's eventually fixed
-      this.labels.eq(0).trigger('mouseover').trigger('mouseenter').find('input').trigger('focus');
+      this.labels.filter(':not(.ui-state-disabled)').eq(0).trigger('mouseover').trigger('mouseenter').find('input').trigger('focus');
 
       button.addClass('ui-state-active');
       this._isOpen = true;