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

(cherry picked from commit 3d43f9c809e42b960be94f2f4490d6d14e063486)

crypto/bn/bn_depr.c

@@ -40,7 +40,7 @@ BIGNUM *BN_generate_prime(BIGNUM *ret, int bits, int safe,
         goto err;
 
     /* we have a prime :-) */
-    return ret;
+    return rnd;
  err:
     BN_free(rnd);
     return NULL;