Add missing socket check in can/bcm release.

[Dave Jones <davej@redhat.com>]

We can get here with a NULL socket argument passed from userspace,
so we need to handle it accordingly.

Signed-off-by: Dave Jones <davej@redhat.com>

diff --git a/net/can/bcm.c b/net/can/bcm.c
index 57b1aed..8a6a05e 100644
--- a/net/can/bcm.c
+++ b/net/can/bcm.c
@@ -1427,9 +1427,14 @@ static int bcm_init(struct sock *sk)
 static int bcm_release(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
-	struct bcm_sock *bo = bcm_sk(sk);
+	struct bcm_sock *bo;
 	struct bcm_op *op, *next;
 
+	if (sk == NULL)
+		return 0;
+
+	bo = bcm_sk(sk);
+
 	/* remove bcm_ops, timer, rx_unregister(), etc. */
 
 	unregister_netdevice_notifier(&bo->notifier);