caif: Do not dereference NULL in chnl_recv_cb()

In net/caif/chnl_net.c::chnl_recv_cb() we call skb_header_pointer()
which may return NULL, but we do not check for a NULL pointer before
dereferencing it.
This patch adds such a NULL check and properly free's allocated memory
and return an error (-EINVAL) on failure - much better than crashing..

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/chnl_net.c

@@ -94,6 +94,10 @@ static int chnl_recv_cb(struct cflayer *layr, struct cfpkt *pkt)
 
 	/* check the version of IP */
 	ip_version = skb_header_pointer(skb, 0, 1, &buf);
+	if (!ip_version) {
+		kfree_skb(skb);
+		return -EINVAL;
+	}
 
 	switch (*ip_version >> 4) {
 	case 4: