Fixed crashing bug for union queries where there was no real tables.


mysql-test/r/group_by.result:
  Added test case
mysql-test/t/group_by.test:
  Added test case
sql/db.opt:
  Removed genrated file
sql/item.cc:
  Handled case when table_list->pos_in_tables is not set. Can only happens when there is no real tables in query