Prevent a use-after-free by not updating an ARP entry that has been

Prevent a use-after-free by not updating an ARP entry that has been
removed from the table.

Currently the storage for L2 addresses is freed when an entry is
removed from the table.  That means that we cannot access this
chunk of memory between RTM_DELETE and rtfree(9).

Note that this doesn't apply to MPLS because the associated storage
is currently released by the last rtfree(9).

ok mikeb@