mini_ssl: 密钥密码短语 char* 存储为 SSL_CTX passwd_cb 用户数据,其生命周期长于 Ruby 字符串

`sslctx_initialize` stores a raw `RSTRING_PTR` of the key passphrase as OpenSSL's `default_passwd_cb` **userdata**, which the `SSL_CTX` retains for its whole lifetime. The String it points into is a fresh, un-retained `Open3.capture3` result, and a passphrase is far under 616 bytes — so its bytes are embedded in the object slot and move when the GC compacts.