caif: Allow cfpkt_extr_head to process empty message

Allow NULL pointer in cfpkt_extr_head in order to
skip past header data.
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/cfpkt_skbuff.c

@@ -144,6 +144,7 @@ int cfpkt_extr_head(struct cfpkt *pkt, void *data, u16 len)
 	}
 	from = skb_pull(skb, len);
 	from -= len;
+	if (data)
 		memcpy(data, from, len);
 	return 0;
 }