Improve coding pattern in Parallel Append code.

The create_append_path code didn't consider that list_concat will
modify it's first argument leading to inconsistent traversal of
resulting list.  In practice, it won't lead to any user-visible bug
but changing it for making the code behave consistently.

Reported-by: Tom Lane
Author: Tom Lane
Reviewed-by: Amit Khandekar and Amit Kapila
Discussion: https://postgr.es/m/32365.1528994120@sss.pgh.pa.us

src/backend/optimizer/util/pathnode.c

@@ -1274,7 +1274,7 @@ create_append_path(PlannerInfo *root,
 	pathnode->first_partial_path = list_length(subpaths);
 	pathnode->subpaths = list_concat(subpaths, partial_subpaths);
 
-	foreach(l, subpaths)
+	foreach(l, pathnode->subpaths)
 	{
 		Path	   *subpath = (Path *) lfirst(l);