In icmp6_rip6_input(), once we have a lock, make sure the inp is
not freed. This can happen since the list traversal and locking
was converted to epoch(9). If the inp is marked "freed", skip it.

This prevents a NULL pointer deref panic in ip6_savecontrol_v4()
trying to access the socket hanging off the inp, which was gone
by the time we got there.

Reported by: andrew
Tested by: andrew
Approved by: re (gjb)