Repair crash with unsortable grouping sets.

If there were multiple grouping sets, none of them empty, all of which
were unsortable, then an oversight in consider_groupingsets_paths led
to a null pointer dereference. Fix, and add a regression test for this
case.

Per report from Dang Minh Huong, though I didn't use their patch.

Backpatch to 10.x where hashed grouping sets were added.

src/backend/optimizer/plan/planner.c

@@ -3997,7 +3997,28 @@ consider_groupingsets_paths(PlannerInfo *root,
 
 		Assert(can_hash);
 
-		if (pathkeys_contained_in(root->group_pathkeys, path->pathkeys))
+		/*
+		 * If the input is coincidentally sorted usefully (which can happen
+		 * even if is_sorted is false, since that only means that our caller
+		 * has set up the sorting for us), then save some hashtable space by
+		 * making use of that. But we need to watch out for degenerate cases:
+		 *
+		 * 1) If there are any empty grouping sets, then group_pathkeys might
+		 * be NIL if all non-empty grouping sets are unsortable. In this case,
+		 * there will be a rollup containing only empty groups, and the
+		 * pathkeys_contained_in test is vacuously true; this is ok.
+		 *
+		 * XXX: the above relies on the fact that group_pathkeys is generated
+		 * from the first rollup. If we add the ability to consider multiple
+		 * sort orders for grouping input, this assumption might fail.
+		 *
+		 * 2) If there are no empty sets and only unsortable sets, then the
+		 * rollups list will be empty (and thus l_start == NULL), and
+		 * group_pathkeys will be NIL; we must ensure that the vacuously-true
+		 * pathkeys_contain_in test doesn't cause us to crash.
+		 */
+		if (l_start != NULL &&
+			pathkeys_contained_in(root->group_pathkeys, path->pathkeys))
 		{
 			unhashed_rollup = lfirst_node(RollupData, l_start);
 			exclude_groups = unhashed_rollup->numGroups;

src/test/regress/expected/groupingsets.out

@@ -1018,6 +1018,18 @@ explain (costs off)
          ->  Values Scan on "*VALUES*"
 (9 rows)
 
+-- unsortable cases
+select unsortable_col, count(*)
+  from gstest4 group by grouping sets ((unsortable_col),(unsortable_col))
+  order by unsortable_col::text;
+ unsortable_col | count 
+----------------+-------
+              1 |     4
+              1 |     4
+              2 |     4
+              2 |     4
+(4 rows)
+
 -- mixed hashable/sortable cases
 select unhashable_col, unsortable_col,
        grouping(unhashable_col, unsortable_col),

src/test/regress/sql/groupingsets.sql

@@ -292,6 +292,11 @@ explain (costs off)
   select a, b, grouping(a,b), array_agg(v order by v)
     from gstest1 group by cube(a,b);
 
+-- unsortable cases
+select unsortable_col, count(*)
+  from gstest4 group by grouping sets ((unsortable_col),(unsortable_col))
+  order by unsortable_col::text;
+
 -- mixed hashable/sortable cases
 select unhashable_col, unsortable_col,
        grouping(unhashable_col, unsortable_col),