all calls of fix_fields() are inspected
(copy of cset which I lost in accidatly tree delete)


mysql-test/r/subselect.result:
  test of BUG#1645
mysql-test/t/subselect.test:
  test of BUG#1645
sql/item_subselect.cc:
  removed passing left_expr through parameters, because it present in class fields
  fixed fix_fields() call
sql/item_subselect.h:
  removed passing left_expr through parameters, because it present in class fields
sql/set_var.cc:
  comments added
sql/sql_select.cc:
  fixed BUG#1645 (thd can be used in fix_fields of subqueries)