ext/openssl/ossl_bn.c: fix implicit conversion

* ext/openssl/ossl_bn.c (ossl_bn_initialize): fix precision loss by
  implicit conversion.

* ext/openssl/ossl_bn.c (ossl_bn_initialize): check Bignum overflow.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40463 b2dd03c8-39d4-4d8f-98ff-823fe69b080e