sifp: fix coverity warning
    
    sssd-1.14.1/src/lib/sifp/sss_sifp_dbus.c:51: check_return:
      Calling "dbus_message_append_args_valist" without checking return value
      (as is done elsewhere 4 out of 5 times).
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>