sql/item_func.cc:
  A fix (bug #6441: Aggregate UDF in multi-table query crashes MySQL when returning multiple rows).
  Do nothing in the udf_handler destructor if not_original flag is set.
sql/item_sum.h:
  A fix (bug #6441: Aggregate UDF in multi-table query crashes MySQL when returning multiple rows).
  Set udf.not_original flag if we create an Item from the existent one.
sql/sql_udf.h:
  A fix (bug #6441: Aggregate UDF in multi-table query crashes MySQL when returning multiple rows).
  not_original flag added.