libvhost-user: fix crash when rings aren't ready

Calling libvhost-user functions like vu_queue_get_avail_bytes() when the
queue doesn't yet have addresses will result in the crashes like the
following:

Program received signal SIGSEGV, Segmentation fault.
0x000055c414112ce4 in vring_avail_idx (vq=0x55c41582fd68, vq=0x55c41582fd68)
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
940            vq->shadow_avail_idx = vq->vring.avail->idx;
(gdb) p vq
$1 = (VuVirtq *) 0x55c41582fd68
(gdb) p vq->vring
$2 = {num = 0, desc = 0x0, avail = 0x0, used = 0x0, log_guest_addr = 0, flags = 0}

    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
No locals.
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:960
        num_heads = <optimized out>
    out_bytes=out_bytes@entry=0x7fffd035d7c4, max_in_bytes=max_in_bytes@entry=0,
    max_out_bytes=max_out_bytes@entry=0) at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:1034

Add a pre-condition checks on vring.avail before accessing it.

Fix documentation and return type of vu_queue_empty() while at it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
