Give pull_var_clause() reject/recurse/return behavior for WindowFuncs too.

All along, this function should have treated WindowFuncs in a manner
similar to Aggrefs, ie with an option whether or not to recurse into them.
By not considering the case, it was always recursing, which is OK for most
callers (although I suspect that the case in prepare_sort_from_pathkeys
might represent a bug).  But now we need return-without-recursing behavior
as well.  There are also more than a few callers that should never see a
WindowFunc, and now we'll get some error checking on that.

src/backend/optimizer/plan/createplan.c

@@ -5307,7 +5307,8 @@ prepare_sort_from_pathkeys(Plan *lefttree, List *pathkeys,
 			 * that we treat Aggrefs as if they were variables; this is
 			 * necessary when attempting to sort the output from an Agg node
 			 * for use in a WindowFunc (since grouping_planner will have
-			 * treated the Aggrefs as variables, too).
+			 * treated the Aggrefs as variables, too).  Likewise, if we find a
+			 * WindowFunc in a sort expression, treat it as a variable.
 			 */
 			Expr	   *sortexpr = NULL;
 
@@ -5336,6 +5337,7 @@ prepare_sort_from_pathkeys(Plan *lefttree, List *pathkeys,
 				sortexpr = em->em_expr;
 				exprvars = pull_var_clause((Node *) sortexpr,
 										   PVC_INCLUDE_AGGREGATES |
+										   PVC_INCLUDE_WINDOWFUNCS |
 										   PVC_INCLUDE_PLACEHOLDERS);
 				foreach(k, exprvars)
 				{