Improve style around manipulation of AppendPath node

As coded, the logic in charge of computing parallel_workers based on the
lookup of the paths would assign AppendPath before making sure that it
is a node of the expected type.  The code was not wrong, but that would
encourage incorrect logic, like looking at the values inside the
AppendPath before checking that it is of the correct type.

Backpatch-through: 17

pg_hint_plan.c

@@ -4768,13 +4768,16 @@ pg_hint_plan_set_rel_pathlist(PlannerInfo * root, RelOptInfo *rel,
 
 		foreach(lc, rel->partial_pathlist)
 		{
-			ListCell *lcp;
-			AppendPath *apath = (AppendPath *) lfirst(lc);
-			int		parallel_workers = 0;
+			ListCell   *lcp;
+			Node	   *path = (Node *) lfirst(lc);
+			AppendPath *apath;
+			int			parallel_workers = 0;
 
-			if (!IsA(apath, AppendPath))
+			if (!IsA(path, AppendPath))
 				continue;
 
+			apath = (AppendPath *) path;
+
 			foreach (lcp, apath->subpaths)
 			{
 				Path *spath = (Path *) lfirst(lcp);