sbus: Check string arguments for valid UTF-8 strings
    
    libdbus abort()s when a string argument is not valid UTF-8. Since the
    arguments sometimes come from untrusted sources, it's better to check
    the string validity explicitly.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    (cherry picked from commit 6b01dae732eedee808f32a9cdd4b5656a9f839c4)