This is precisely why the use of goto is frowned upon. Just use proper if statement control flow.

Code:
on *:text:!youtube*:#:{
  if ($2-) {
    if ($nick == %admin. [ $+ [ # ] ]) || ($nick == %superadmin) {
      set %youtube. $+ # $2-
      msg # New youtube message has been set.
    }
  }
  else {
    msg # %youtube. [ $+ [ # ] ]
  }
}