aco/ngg: Add assertion to make sure we always know the vertex count.

Just a sanity check to avoid hangs caused by missing this
in the future.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7213>
2 files changed