Fix failure to detect some cases of improperly-nested aggregates.

check_agg_arguments_walker() supposed that it needn't descend into
the arguments of a lower-level aggregate function, but this is
just wrong in the presence of multiple levels of sub-select.  The
oversight would lead to executor failures on queries that should
be rejected.  (Prior to v11, they actually were rejected, thanks
to a "redundant" execution-time check.)

Per bug #17835 from Anban Company.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/17835-4f29f3098b2d0ba4@postgresql.org

src/backend/parser/parse_agg.c

@@ -695,8 +695,7 @@ check_agg_arguments_walker(Node *node,
 				context->min_agglevel > agglevelsup)
 				context->min_agglevel = agglevelsup;
 		}
-		/* no need to examine args of the inner aggregate */
-		return false;
+		/* Continue and descend into subtree */
 	}
 	if (IsA(node, GroupingFunc))
 	{

src/test/regress/expected/aggregates.out

@@ -954,6 +954,12 @@ select (select max(min(unique1)) from int8_tbl) from tenk1;
 ERROR:  aggregate function calls cannot be nested
 LINE 1: select (select max(min(unique1)) from int8_tbl) from tenk1;
                            ^
+select avg((select avg(a1.col1 order by (select avg(a2.col2) from tenk1 a3))
+            from tenk1 a1(col1)))
+from tenk1 a2(col2);
+ERROR:  aggregate function calls cannot be nested
+LINE 1: select avg((select avg(a1.col1 order by (select avg(a2.col2)...
+                                                        ^
 --
 -- Test removal of redundant GROUP BY columns
 --

src/test/regress/sql/aggregates.sql

@@ -334,6 +334,9 @@ drop table minmaxtest cascade;
 -- check for correct detection of nested-aggregate errors
 select max(min(unique1)) from tenk1;
 select (select max(min(unique1)) from int8_tbl) from tenk1;
+select avg((select avg(a1.col1 order by (select avg(a2.col2) from tenk1 a3))
+            from tenk1 a1(col1)))
+from tenk1 a2(col2);
 
 --
 -- Test removal of redundant GROUP BY columns