prime: Avoid rare assertion failure in gcry_prime_check.

* cipher/primegen.c (is_prime): Don't fail on the assert X > 1.

When using gcry_prime_check the function is_prime can be called with
quite small candidates so there is a real chance that the random X
values is indeed 0 or 1. This would trigger the assert. To avoid
this we now retry in this case.

• Reported-by: Heiko Stamer
• Signed-off-by: Werner Koch <wk@gnupg.org>