tls: do not free cert in `.getCertificate()`

The documentation of `SSL_get_certificate` states that it returns
an internal pointer that must not be freed by the caller.

Therefore, using a smart pointer to take ownership is incorrect.

Refs: https://man.openbsd.org/SSL_get_certificate.3
Refs: https://github.com/nodejs/node/pull/24261
Fixes: https://github.com/nodejs-private/security/issues/217

PR-URL: https://github.com/nodejs/node/pull/25490


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>