Error in creativecommons_license_sort()

'admin/config/search/creativecommons' gives the following error several times:
* Notice: Undefined index: mark in creativecommons_license_sort() (line 854 of /public_html/sites/all/modules/creativecommons/creativecommons.module).

The 'publicdomain' license needs to be called 'mark' instead. However, the licenses may change again, so we should do a check before we run into an error again.