Fix PARSE position capture combined with SET or COPY

The recent extension to allow set-word! usage in SET and COPY rules introduced a regression: set-word!s (for position capture) in the rule immediately following a `SET word` or `COPY word` rule cause a match failure. For example:

    >> parse [x] [set val pos: word!]
    == false  ;; Expected: true

This commit fixes this issue (CureCode issue 2130), a regression caused by 8db79a9 (PR #165).

Corresponding tests have been added to the test suite (rebolsource/rebol-test@fe8df53).