st/mesa: treat resource-less xfb buffers as if they weren't there

st/mesa: treat resource-less xfb buffers as if they weren't there

If a transform feedback buffer's size is 0, st_bufferobj_data doesn't
end up creating a buffer for it. There's no point in trying to write to
such a buffer, so just pretend as if it's not really there.

This fixes arb_gpu_shader5-xfb-streams-without-invocations on nvc0.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "10.4 10.5" <mesa-stable@lists.freedesktop.org>