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.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.188.4.1 -> r1.188.4.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.188.4.1&r2=1.188.4.2)