MDEV-18015: Assertion `global_status_var.global_memory_used == 0' failed when using UDF , window functions and views

Adding destructor for Group_bound_tracker to free Cached_item_str.
The Cached_item for window functions are allocated on THD:mem_root
but the Cached_item_str has value of type string which is allocated on
the heap, so we need to call free() for it