eZCollaborationItemGroupLink: Changed wrong var comparison to assignments

When `$userID` is not provided (or `false` is given) to the methods `fetch()` and `fetchList()` the current user's ID should be used. Currently, `$userID` would remain `false` because `$userID` is compared to `eZUser::currentUserID();` instead that the latter is assigned to the former.

Cheers
:octocat: Jérôme