Fix bn_gcd code to check return value when calling BN_one()

BN_one() uses the expand function which calls malloc which may fail.
All other places that reference BN_one() check the return value.

The issue is triggered by a memory allocation failure.
Detected by PR #18355

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from openssl/openssl#18697)