pgsql: Fix failure with whole-row reference to a subquery.

Fix failure with whole-row reference to a subquery.

Simple oversight in commit 1cb108efb0e60d87e4adec38e7636b6e8efbeb57 ---
recursively examining a subquery output column is only sane if the
original Var refers to a single output column.  Found by Kevin Grittner.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e243bd79d98ff4dc5acc20a290dbdc1ad2e17e91

Modified Files
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src/backend/utils/adt/selfuncs.c |    6 ++++++
1 file changed, 6 insertions(+)