gnutls: improve safety in handshake thread retrieve function
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.

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking