ieee802154: lowpan_header_create check must check daddr

Packet sockets may call dev_header_parse with NULL daddr. Make
lowpan_header_ops.create fail.

Fixes: 87a93e4e ("ieee802154: change needed headroom/tailroom")
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Acked-by: NAlexander Aring <aring@mojatatu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan/tx.c

@@ -48,6 +48,9 @@ int lowpan_header_create(struct sk_buff *skb, struct net_device *ldev,
 	const struct ipv6hdr *hdr = ipv6_hdr(skb);
 	struct neighbour *n;
 
+	if (!daddr)
+		return -EINVAL;
+
 	/* TODO:
 	 * if this package isn't ipv6 one, where should it be routed?
 	 */