MySQL Bug #12408412: GROUP_CONCAT + ORDER BY + INPUT/OUTPUT SAME USER VARIABLE = CRASH
and
MySQL Bug#14664077 SEVERE PERFORMANCE DEGRADATION IN SOME CASES WHEN USER VARIABLES ARE USED


sql/item_func.cc:
  don't use anything from Item_func_set_user_var::fix_fields()
  in Item_func_set_user_var::save_item_result()
sql/sql_class.cc:
  Call suv->save_item_result(item) *before* doing suv->fix_fields(), because
  the former evaluates the item (and caches its value), while the latter marks
  the user variable as non-const. The problem is that the item was fix_field'ed
  when the user variable was const, and it doesn't expect it to change to non-const
  in the middle of the execution.