Fix logic error in add_paths_to_partial_grouping_rel.

Commit 3bf05e096b sometimes uses the
cheapest_partial_path variable in this function to mean the cheapest
one from the input rel and at other times the cheapest one from the
partially grouped rel, but it never resets it, so we can end up with
bad plans, leading to "ERROR: Aggref found in non-Agg plan node".

Jeevan Chalke, per a report from Andreas Joseph Krogh and a separate
off-list report from Rajkumar Raghuwanshi

Discussion: http://postgr.es/m/CAM2+6=X9kxQoL2ZqZ00E6asBt9z+rfyWbOmhXJ0+8fPAyMZ9Jg@mail.gmail.com

src/backend/optimizer/plan/planner.c

@@ -6339,6 +6339,9 @@ add_paths_to_partial_grouping_rel(PlannerInfo *root,
 	 */
 	generate_gather_paths(root, partially_grouped_rel, true);
 
+	/* Get cheapest partial path from partially_grouped_rel */
+	cheapest_partial_path = linitial(partially_grouped_rel->partial_pathlist);
+
 	/*
 	 * generate_gather_paths won't consider sorting the cheapest path to match
 	 * the group keys and then applying a Gather Merge node to the result;