tipc: Fix NULL pointer dereference in __tipc_sendstream()

tipc_sendstream() may send zero length packet, then tipc_msg_append()
do not alloc skb, skb_peek_tail() will get NULL, msg_set_ack_required
will trigger NULL pointer dereference.

Reported-by:  <syzbot+8eac6d030e7807c21d32@syzkaller.appspotmail.com>
Fixes: 0a3e060f ("tipc: add test for Nagle algorithm effectiveness")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>