Fix the memory leak in `X509Builder::append_extension`.

    }

    /// Adds an X509 extension value to the certificate.

    ///

    /// This works just as `append_extension` except it takes ownership of the `X509Extension`.

    pub fn append_extension(&mut self, extension: X509Extension) -> Result<(), ErrorStack> {

        self.append_extension2(&extension)

    }

    /// Adds an X509 extension value to the certificate.

    ///

    /// This corresponds to [`X509_add_ext`].

    ///

    /// [`X509_add_ext`]: https://www.openssl.org/docs/man1.1.0/man3/X509_get_ext.html

    pub fn append_extension2(&mut self, extension: &X509ExtensionRef) -> Result<(), ErrorStack> {

        unsafe {

            cvt(ffi::X509_add_ext(self.0.as_ptr(), extension.as_ptr(), -1))?;

            mem::forget(extension);

            Ok(())

        }

    }