Fix segfault in openssl x509 -modulus

Fix segfault in openssl x509 -modulus

The command ``openssl x509 -noout -modulus -in cert.pem`` used to segfaults
sometimes because an uninitialized variable was passed to
``BN_lebin2bn``. The bug triggered an assertion in bn_expand_internal().

Fixes: https://github.com/openssl/openssl/issues/15899
Signed-off-by: Christian Heimes <christian@python.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15900)