crypto/bn: fix return value in BN_generate_prime

When the ret parameter is NULL the generated prime
is in rnd variable and not in ret.

CLA: trivial

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8076)

(cherry picked from commit 3d43f9c8)