MDEV-34227 On startup: UBSAN: runtime error: applying non-zero offset in...

MDEV-34227 On startup: UBSAN: runtime error: applying non-zero offset in JOIN::make_aggr_tables_info in sql/sql_select.cc

Avoid undefined behaviour (applying offset to nullptr).
The reported scenario is covered in mysql-test/connect-no-db.test
No new tests needed.

sql/sql_select.cc

@@ -3325,7 +3325,8 @@ bool JOIN::make_aggr_tables_info()
 {
   List<Item> *curr_all_fields= &all_fields;
   List<Item> *curr_fields_list= &fields_list;
-  JOIN_TAB *curr_tab= join_tab + const_tables;
+  // Avoid UB (applying .. offset to nullptr) when join_tab is nullptr
+  JOIN_TAB *curr_tab= join_tab ? join_tab + const_tables : nullptr;
   TABLE *exec_tmp_table= NULL;
   bool distinct= false;
   bool keep_row_order= false;
@@ -3883,9 +3884,9 @@ bool JOIN::make_aggr_tables_info()
     - duplicate value removal
     Both of these operations are done after window function computation step.
   */
-  curr_tab= join_tab + total_join_tab_cnt();
   if (select_lex->window_funcs.elements)
   {
+    curr_tab= join_tab + total_join_tab_cnt();
     if (!(curr_tab->window_funcs_step= new Window_funcs_computation))
       DBUG_RETURN(true);
     if (curr_tab->window_funcs_step->setup(thd, &select_lex->window_funcs,