net/ieee802154: reject zero-sized raw_sendmsg()

syzbot is hitting skb_assert_len() warning at raw_sendmsg() for ieee802154
socket. What commit dc633700f0 ("net/af_packet: check len when
min_header_len equals to 0") does also applies to ieee802154 socket.

Link: https://syzkaller.appspot.com/bug?extid=5ea725c25d06fb9114c4
Reported-by: syzbot <syzbot+5ea725c25d06fb9114c4@syzkaller.appspotmail.com>
Fixes: fd18942244 ("bpf: Don't redirect packets with invalid pkt_len")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/socket.c

@@ -251,6 +251,9 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 		return -EOPNOTSUPP;
 	}
 
+	if (!size)
+		return -EINVAL;
+
 	lock_sock(sk);
 	if (!sk->sk_bound_dev_if)
 		dev = dev_getfirstbyhwtype(sock_net(sk), ARPHRD_IEEE802154);