How to redirect to user organic group on login
Since you are using code to do this you should be able to use og_get_groups_by_user to return that information. This will return all groups of either the active user or the $account object that you pass to the function. 

You can also use Rules to do this without code pretty easily. There is a rule export on this page to help get started if you want to go that way.

Just to add: the place you are looking for the og group should be under og_membership I think, not og_user_nodes but that's off the top of my head :)