Problem:
  item->name was NULL for Item_user_var_as_out_param
  which made strcmp(something, item->name) crash in the LOAD XML code.
  
  Fix:
  - item_func.h: Adding set_name() in constuctor for Item_user_var_as_out_param
  - sql_load.cc: Changing the condition in write_execute_load_query_log_event() which
  distiguished between Item_user_var_as_out_param and Item_field
  from
    if (item->name == NULL)
  to
    if (item->type() == Item::FIELD_ITEM)
  - loadxml.result, loadxml.test: adding tests