nir/constant_folding: init nir_const_value to zero

To avoid NIR validation failures.

Fixes: 9df1ff3678 ("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>

src/compiler/nir/nir_opt_constant_folding.c

@@ -230,6 +230,7 @@ try_fold_intrinsic(nir_builder *b, nir_intrinsic_instr *instr,
                                 instr->dest.ssa.bit_size);
       } else {
          nir_const_value imm[NIR_MAX_VEC_COMPONENTS];
+         memset(imm, 0, sizeof(imm));
          uint8_t *data = (uint8_t*)b->shader->constant_data + base;
          for (unsigned i = 0; i < instr->num_components; i++) {
             unsigned bytes = instr->dest.ssa.bit_size / 8;