Fix parallel index builds for dynamic_shared_memory_type=none.

The previous code failed to realize that this setting effectively
disables parallelism, and would crash if it decided to attempt
parallelism anyway.  Instead, treat it as a disabling condition.

Kyotaro Horiguchi, who also reported the issue. Reviewed by Michael
Paquier and Peter Geoghegan.

Discussion: http://postgr.es/m/20180209.170635.256350357.horiguchi.kyotaro@lab.ntt.co.jp

src/backend/optimizer/plan/planner.c

@@ -5825,7 +5825,8 @@ plan_create_index_workers(Oid tableOid, Oid indexOid)
 	double		allvisfrac;
 
 	/* Return immediately when parallelism disabled */
-	if (max_parallel_maintenance_workers == 0)
+	if (dynamic_shared_memory_type == DSM_IMPL_NONE ||
+		max_parallel_maintenance_workers == 0)
 		return 0;
 
 	/* Set up largely-dummy planner state */