fix usage of default options

`$.ui.multiselect.defaults.dividerLocation` can be used in old versions of jQuery UI only and  `$.ui.multiselect.prototype.options.dividerLocation` in new one. The above fix should fix exception in case if `dividerLocation` in not defixed in input options. In the case `$.ui.multiselect.defaults` will undefined and `$.ui.multiselect.defaults.dividerLocation` produces exception.