ossl_bn.c: fix ossl_bn_initialize bug with integer

* ext/openssl/ossl_bn.c (ossl_bn_initialize): fix buffer overflow on
  x64 Windows and memory leak when initializing with integer.
  [ruby-core:54615] [Bug #8337]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40513 b2dd03c8-39d4-4d8f-98ff-823fe69b080e