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 <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
Reviewed-by: Dmitry Belyavskiy <38c64d6e24766247aad56af3a6ddb0056ad36a44@gmail.com>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15900)

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