af-packet: fix oops when socket is not present

Due to a NULL dereference, the following patch is causing oops
in normal trafic condition:

commit c0de08d0
Author: Eric Leblond <eric@regit.org>
Date:   Thu Aug 16 22:02:58 2012 +0000

    af_packet: don't emit packet on orig fanout group

This buggy patch was a feature fix and has reached most stable
branches.

When skb->sk is NULL and when packet fanout is used, there is a
crash in match_fanout_group where skb->sk is accessed.
This patch fixes the issue by returning false as soon as the
socket is NULL: this correspond to the wanted behavior because
the kernel as to resend the skb to all the listening socket in
this case.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: David S. Miller <davem@davemloft.net>