Function emsa_pss_get_salt_len 

fn emsa_pss_get_salt_len(
    db: &[u8],
    em_len: usize,
    h_len: usize,
) -> (usize, Choice)

Detect salt length by scanning DB for the 0x01 separator byte. Returns (s_len, valid) where s_len is 0 on failure.