pf: Fix panic on overlapping interface names

In rare situations[*] it's possible for two different interfaces to have
the same name. This confuses pf, because kifs are indexed by name (which
is assumed to be unique). As a result we can end up trying to
if_rele(NULL), which panics.

Explicitly checking the ifp pointer before if_rele() prevents the panic.
Note pf will likely behave in unexpected ways on the the overlapping
interfaces.

• Insert an interface in a vnet jail. Rename it to an interface which

exists on the host. Remove the jail. There are now two interfaces with
the same name in the host.