Fix error handling in bn_exp
Fix error handling in bn_exp

In the event of an error |rr| could be NULL. Therefore don't assume you can
use |rr| in the error handling code.

Reviewed-by: Andy Polyakov <015edf36635e030d5e009e08808a59984e629740@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl