Attempt to resolve FOR SYSTEM_TIME expression as field for derived
table is done before derived table is fully prepared, so we fail on
assertion that table_list->table is missing.

Actually Vers_history_point::resolve_unit() is done under the call of
mysql_derived_prepare() itself (sql_derived.cc:824) and the table is
assigned later at 867.

The fix disables unit resolution for field type in FOR SYSTEM_TIME
expression as it does a little sense in any case: making historical
queries based on variable field values produces the result of multiple
time points.

fix_fields_if_needed() in resolve_units() was introduced by 46be3198