MDEV-31743 Server crash in store_length, assertion failure in Type_handler_string_result::sort_length

After MDEV-21580 the truncation of SORT_FIELD::length
  set_if_smaller(sortorder->length, thd->variables.max_sort_length)

became conditional:
  if (is_variable_sized())
    set_if_smaller(length, thd->variables.max_sort_length)

To provide correct functioning of is_variable_sized() SORT_FIELD::type
must be set properly. This commit adds the necessary initialization
of SORT_FIELD::type to JOIN_TAB::remove_duplicates() as it is done
in filesort's sortlength() function.

DBUG_ASSERT is added to sortlength() just in case to prevent
a possible uint32 overflow