can: Add missing socket check in can/bcm release.

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

Signed-off-by: Dave Jones <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/can/bcm.c

···
 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. */