Fix memory leak in BN_rand_range()

Fix memory leak in BN_rand_range()

The patch enables BN_rand_range() to exit immediately
if BIGNUM *rnd is NULL.

CLA: trivial

Fixes: #18951
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18982)

(cherry picked from commit 70f589ae41928edda18470ba1c3df82af02a92b3)