gnutls: improve safety in handshake thread retrieve function

Coverity noticed that if g_tls_connection_base_handshake_thread_request_certificate() returns FALSE here, we call g_tls_certificate_gnutls_copy_free() twice with the same data. It's obvious and should never have happened. Oops.

(WIP: status is only due to hard code freeze. This is ready for after freeze.)