hw/xen: Check if len is 0 before memcpy()

hw/xen: Check if len is 0 before memcpy()

data->data can be NULL when len is 0. Strictly speaking, the behavior
of memcpy() in such a scenario is undefined so UBSan complaints.

Satisfy UBSan by checking if len is 0 before memcpy().

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>