BROWSE: Return R_UNSET rather than R_NONE.

Following discussion on AltMe, modify `browse` to return `R_UNSET` rather than `R_NONE`.

In addition, handle the `none` case correctly (return early). At present, browsing `none`yields a SEGFAULT; this commit addresses Cure Code ticket #1991 ( http://curecode.org/rebol3/ticket.rsp?id=1991 ).