Try and silence spurious Coverity warning.

gset_data (aka gd) in planner.c is always non-null if and only if
parse->groupingSets is non-null, but Coverity doesn't know that and
complains.  Feed it an assertion to see if that keeps it happy.

Author: RhodiumToad

src/backend/optimizer/plan/planner.c

@@ -3362,6 +3362,8 @@ get_number_of_groups(PlannerInfo *root,
 			ListCell   *lc;
 			ListCell   *lc2;
 
+			Assert(gd);  /* keep Coverity happy */
+
 			dNumGroups = 0;
 
 			foreach(lc, gd->rollups)