net/ieee802154: reject zero-sized raw_sendmsg()

syzbot is hitting skb_assert_len() warning at raw_sendmsg() for ieee802154
socket. What commit dc633700

 ("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: fd189422

 ("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>