st/mesa: don't draw instead of asserting in transform feedback

st/mesa: don't draw instead of asserting in transform feedback

if we get a request to take the count from feedback, but there
is no buffer to take it from, just draw as if we got 0 vertices
so nothing.

This fixes this assert killing the ogl conform, and a piglit
test I've sent.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>