For structs / arrays, json_to_vm_value() calls json_load_page() on
freshly allocated pages for which no constructor has been called.
json_load_page() should not call destructors for such uninitialized
pages.
json_load_page() is also called by BanMisc.LoadStruct and File.Read,
with an initialized struct. In that case destructors should be called.
This fixes crash in Widenyo after system.GroupLoad.