make st_select_lex::setup_ref_array() take into account that 
Item_sum-descendant objects located within descendant SELECTs
may be added into ref_pointer_array.


sql/item_sum.cc:
  BUG#21477 "memory overruns for certain kinds of subqueries":
  Make SELECT_LEX::n_sum_items contain # of Item_sum-derived objects
  that exist within this SELECT.
sql/sql_lex.h:
  BUG#21477 "memory overruns for certain kinds of subqueries":
  Add SELECT_LEX::n_sum_items and SELECT_LEXT::n_child_sum_items.
sql/sql_yacc.yy:
  BUG#21477 "memory overruns for certain kinds of subqueries":
  Make SELECT_LEX::n_sum_items contain # of Item_sum-derived objects
  that exist within this SELECT.