Fix failure with whole-row reference to a subquery.

Simple oversight in commit 1cb108ef ---
recursively examining a subquery output column is only sane if the
original Var refers to a single output column.  Found by Kevin Grittner.

src/backend/utils/adt/selfuncs.c

@@ -4502,6 +4502,12 @@ examine_simple_variable(PlannerInfo *root, Var *var,
 		RelOptInfo *rel;
 		TargetEntry *ste;
 
+		/*
+		 * Punt if it's a whole-row var rather than a plain column reference.
+		 */
+		if (var->varattno == InvalidAttrNumber)
+			return;
+
 		/*
 		 * Punt if subquery uses set operations or GROUP BY, as these will
 		 * mash underlying columns' stats beyond recognition.  (Set ops are