Add filter for $_GET param "jsop"

A security team we worked with on a recent project requested that we patch this module, and we're looking to see if this should get rolled into a release and if there's any feedback on the request for this change.

The update includes the following change to imce.page.inc, line 13:

$jsop = isset($_GET['jsop']) ? $_GET['jsop'] : NULL;

to:

$jsop = isset($_GET['jsop']) ? filter_xss($_GET['jsop']) : NULL;