Bug#16192219 CRASH IN TEST_IF_SKIP_SORT_ORDER ON SELECT DISTINCT WITH ORDER BY

This is a backport of the fix for:

Bug#13633549 HANDLE_FATAL_SIGNAL IN TEST_IF_SKIP_SORT_ORDER/CREATE_SORT_INDEX
Don't invoke the range optimizer for a NULL select.

sql/sql_select.cc

@@ -13753,7 +13753,8 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
 
     if (best_key >= 0)
     {
-      if (table->quick_keys.is_set(best_key) && best_key != ref_key)
+      if (select &&
+          table->quick_keys.is_set(best_key) && best_key != ref_key)
       {
         key_map map;
         map.clear_all();       // Force the creation of quick select