ec: avoid unintentional condition branches for 25519, 448 and 256k1
* mpi/ec.c (ec_addm_25519, ec_subm_25519, ec_mulm_25519, ec_addm_448) (ec_subm_448, ec_mulm_448, ec_secp256k1_mod): Use mpih_limb_is_zero and mpih_limb_is_not_zero instead of comparison to zero.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>