Fix testing of parallel-safety of scan/join target.

In commit 960df2a9 ("Correctly assess parallel-safety of tlists when
SRFs are used."), the testing of scan/join target was done incorrectly,
which caused a plan-quality problem.  Backpatch through to v11 where
the aforementioned commit went in, since this is a regression from v10.

Author: Etsuro Fujita
Reviewed-by: Robert Haas and Tom Lane
Discussion: https://postgr.es/m/5C75303E.8020303@lab.ntt.co.jp

src/backend/optimizer/plan/planner.c

@@ -1993,7 +1993,7 @@ grouping_planner(PlannerInfo *root, bool inheritance_update,
 		{
 			scanjoin_target = make_group_input_target(root, final_target);
 			scanjoin_target_parallel_safe =
-				is_parallel_safe(root, (Node *) grouping_target->exprs);
+				is_parallel_safe(root, (Node *) scanjoin_target->exprs);
 		}
 		else
 		{