Fix panic during lagg destruction with simultaneous status check

If you run "ifconfig lagg0 destroy" and "ifconfig lagg0" at the same time a
page fault may result. The first process will destroy ifp->if_lagg in
lagg_clone_destroy (called by if_clone_destroy). Then the second process
will observe that ifp->if_lagg is NULL at the top of lagg_port_ioctl and
goto fallback: where it will promptly dereference ifp->if_lagg anyway.

The solution is to repeat the NULL check for ifp->if_lagg

MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D8512