Fix race between ifconfig destroy and ARP timer.

Fix race between ifconfig destroy and ARP timer.

After if_detach() has called if_remove(), if_get() will return NULL.
Before if_detach() grabs the net lock, ARP timer can still run.  In
this case arptfree() should just return, instead of triggering an
assertion because ifp is NULL.  The ARP route will be deleted later
when in_ifdetach() calls in_purgeaddr().

OK kn@ mvs@ claudio@