Added test case for bug #4767.
item_sum.cc:
  Added a correct setting of the maybe_null flag for a copy of 
  an Item_sum object where the argument was a field of an inner table
  in an outer join read from a temporary table.
  It's part of the fix for bug #4767.
sql_select.cc:
  Made change_refs_to_tmp_fields work correctly
  for test case of bug #4767 where Item_sum::get_tmp_table_item
  failed to build a correct copy of an Item_sum object referring
  to a field in a temporary table.
  It looks like a hack yet.