Free indexed_tlist memory explicitly within set_plan_refs()

While trying to fix a largely unrelated bug, I noticed that the new
build_tlist_index() call for the "excluded" targetlist (used by ON
CONFLICT DO UPDATE queries) does not have its memory subsequently
freed by the caller. Since every other call to build_tlist_index()
does this, and comments above build_tlist_index() encourage it, I
think the new caller should do the same.

Attached patch adds such a pfree() call.
--
Peter Geoghegan