Redundant primality test in gen_safe_prime

[Milap Sheth]

The function to generate a safe prime, gen_safe_prime, performs another primality test on the random prime that was just generated.

Specifically, in gen_safe_prime, gen_prime returns a probable prime, but the next check _is_safe_prime(...) && lucas(...) tests again that candidate is a prime. This is already done in gen_prime, so we now only need to check if (candidate - 1)/2 is also prime.