mpiutil: use global vone and vzero
* mpi/mpiutil.c (_gcry_mpi_set_cond, _gcry_mpi_swap_cond): Use _gcry_ct_vzero and _gcry_ct_vone.
Cherry pick master commit of:
d4aee9ace9a904446b987dddc2999119c4d62dae
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>