ossl_bn.c: reduce alloca/malloc

* ext/openssl/ossl_bn.c (ossl_bn_initialize): no need of alloca for
  small fixed size array.
* ext/openssl/ossl_bn.c (ossl_bn_initialize): check overflow first,
  and use alloca for small size input.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40518 b2dd03c8-39d4-4d8f-98ff-823fe69b080e