When the SELECT query from a trigger that used a subquery
in its SELECT list was prepared the counter select_n_having_items
was incremented in the constructor Item::Item(THD *thd).
As a result each invocation of the trigger required more and more
memory for the ref_pointer_array for this SELECT.
Made sure that the counter st_select_lex::select_n_having_items
would be incremented only at the first execution of such trigger.