netrom: Kill spurious NULL'ing of sk->sk_socket.

In nr_release(), one code path calls sock_orphan() which
will NULL out sk->sk_socket already.

In the other case, handling states other than NR_STATE_{0,1,2,3},
seems to not be possible other than due to bugs.  Even for an
uninitialized nr->state value, that would be zero or NR_STATE_0.
It might be wise to stick a WARN_ON() here.

Signed-off-by: David S. Miller <davem@davemloft.net>