MDEV-29924 Assertion `(((nr) % (1LL << 24)) % (int) log_10_int[6 - dec]) == 0' failed in my_time_packed_to_binary on SELECT when using TIME field

when assigning the cached item to the Item_cache for the first time
make sure to use Item_cache::setup(), not Item_cache::store().
Because the former copies the metadata (and allocates memory, in case
of Item_cache_row), and Item_cache::decimal must be set for
comparisons to work correctly.