ubusd: fix incomplete copy of shared buf during queue-ing

For a shared ubus_msg_buf, the ubus_msg_ref function will
create a copy for queue-ing.

Problem is, that during the dequeue (especially) in client_cb,
the header is 0-ed (because it's was a newly alloc-ed buffer).

And during ubus_msg_writev(), the header info will be ignored
by the client.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>