sntrup761: harden mask generation against branch optimization
* cipher/sntrup761.c (int32_MINMAX, uint32_divmod_uint14) (int32_divmod_uint14, int16_t_nonzero_mask, int16_t_negative_mask) (Ciphertexts_diff_mask): Generate mask with const-time helper function.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>