fixed cols() method call (it have to be called only after fix_fields())


mysql-test/r/subselect.result:
  Comparison subquery with * and row
mysql-test/t/subselect.test:
  Comparison subquery with * and row
sql/item_cmpfunc.h:
  initialization allowed_arg_cols for autodetection
sql/item_func.cc:
  support of allowed_arg_cols autodetection by first argument
sql/item_func.h:
  commant
sql/item_subselect.cc:
  correct column number fetching for subqueries
sql/sql_lex.h:
  method to check that UNION is prepared