An aggregate function reference was resolved incorrectly and
caused a crash in count_field_types.
 Must use real_item() to get to the real Item instance through
the reference


mysql-test/r/func_group.result:
  Bug #20868: Client connection is broken on SQL query error
   * test case for the bug
mysql-test/t/func_group.test:
  Bug #20868: Client connection is broken on SQL query error
   * test case for the bug
sql/sql_select.cc:
  Bug #20868: Client connection is broken on SQL query error
   * correctly resolve aggregate function references.