Passing in invalid data to listbox causes 500 error

The current gl_redirect_listbox_tag implementation does not allow for the selected param to be anything other than a valid element in items or nil. If someone manually modifies the URL to pass in a value not in items, a 500 error occurs. We should fail gracefully and could fall back to the first item in items, which is the current behavior if nil is passed in.

This has come up so far in !80681 (comment 862019211) and !81370 (comment 854661919).