vhost_net: zerocopy: fix possible NULL pointer dereference of vq->bufs

When we want to disable vhost_net backend while there's a tx work, a possible
NULL pointer defernece may happen we we try to deference the vq->bufs after
vhost_net_set_backend() assign a NULL to it.

As suggested by Michael, fix this by checking the vq->bufs instead of
vhost_sock_zcopy().

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>