[Rd] build fails with --enable-strict-barrier

[coypu at sdf.org]

Hi, I'm very new here and unfamiliar with R.
However, I feel like I can still provide some input.

This is a bug, it must not use DATAPTR.
Perhaps this is a suitable replacement for these instances:
xd = x[1];

A safer temporary option is to drop this flag.

I am unsure whether that is acceptable though.