vhost_net: add an assertion for TAP client backends

An assertion was missing for tap vhost backends that enforces a non-null
reference from get_vhost_net(). Both vhost-net-user and vhost-net-vdpa
enforces this. Enforce the same for tap. Unit tests pass with this change.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Message-Id: <20230619041501.111655-1-anisinha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>