MFC r367594:

MFC r367594:
  Fix possible NULL pointer dereference.

  lagg(4) replaces if_output method of its child interfaces and expects
  that this method can be called only by child interfaces. But it is
  possible that lagg_port_output() could be called by children of child
  interfaces. In this case ifnet's if_lagg field is NULL. Add check that
  lp is not NULL.