pgsql: Check for out-of-range varoattno in deparse_context_for_subplan.

Log Message:
-----------
Check for out-of-range varoattno in deparse_context_for_subplan.
I have seen this case in CVS tip due to new "physical tlist" optimization
for subqueries.  I believe it probably can't happen in existing releases,
but the check is not going to hurt anything, so backpatch to 8.0 just
in case.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.203 -> r1.204)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.203&r2=1.204)