nir/constant_folding: init nir_const_value to zero

To avoid NIR validation failures.

Fixes: 9df1ff36781 ("nir/constant_folding: Use the builder")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7035>
1 file changed