nir_to_tgsi: Require the block index to always be populated

In some cases like when using `NIR_DEBUG=serialize`, impl->num_blocks is 0
which leads to assertions error in the blocklist. Make sure to require the
num_blocks to be populated.

Fixes: 74c02d99b224fbc02132b54bf23e6ac1e117d805

Signed-off-by: Corentin Noël <corentin.noel@collabora.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15640>
1 file changed