Bug#14683676 ENDLESS MEMORY CONSUMPTION IN SETUP_REF_ARRAY WITH MAX IN SUBQUERY

n_child_sum_items kept increasing.
Since it is used for calculating the size of ref_pointer_array,
we will allocate larger and larger chunks of memory, until we hit some
operating system limit.
The memory is free()d at disconnect, but is most likely *not*
returned to the operating system.

sql/sql_lex.cc

@@ -1616,6 +1616,7 @@ void st_select_lex::init_query()
   ref_pointer_array= 0;
   select_n_where_fields= 0;
   select_n_having_items= 0;
+  n_child_sum_items= 0;
   subquery_in_having= explicit_limit= 0;
   is_item_list_lookup= 0;
   first_execution= 1;