Need isset check for $choices['league_interest']

Was getting:
Notice: Undefined index: interest_groups in _mailchimp_lists_subscription_has_changed() (line 337
and
Warning: Invalid argument supplied for foreach() in _mailchimp_lists_subscription_has_changed() (line 337

Simple enough fix, just need to wrap all of that foreach ($choices['interest_groups'] as $interest_group) loop in if (isset($choices['interest_groups']))