MDEV-31113 Server crashes in store_length / Type_handler_string_result::make_sort_key with DISTINCT and group function

Fix-up for commit 476b24d0
  Author: Monty
  Date:   Thu Feb 16 14:19:33 2023 +0200
    MDEV-20057 Distinct SUM on CROSS JOIN and grouped returns wrong result
which misses initializing of sorder->suffix_length.
In this commit the initialization is implemented by passing
MY_ZEROFILL flag to the allocation of SORT_FIELD elements