Adding a group to an interface is protected by the 'if_addr_lock' mutex,
but reading is by using epoch. It thus may be possible for
if_getgroup(), called under 'net_epoch_preempt', to observe a 'struct
ifg_list' holding a group added to an interface, but without that
structure being fully initialized (in particular, 'igl_group' may still
be observed as being NULL).
Reported by: bz
Fixes: d7c5a620e2b9 ("ifnet: Replace if_addr_lock rwlock with epoch + mutex")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation