fix compilation of x[y,]

Oh boy.  I do not pretend to totally understand what is going on, but
what seemed to be happening is that somehow when walking the parse
tree to adjust srcrefs to the real file position rather than the
string position, the `empty' space in x[y,] was turning from a
zero-element name to a missing object, and then subsequent attempts to
evaluate the missing object (or even return it) were failing.

The workaround is to short-circuit the process for name objects, which
are atomic and (empirically) do not have srcrefs attached anyway and
so can be returned without modification.