The string returned by g_win32_error_message() has to be
deallocated with g_free().

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200228100726.8414-5-philmd@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>