make items rot even inside fridges

`fridge` is set to 0 at the end of `item::rotten` and `last_rot_check` to `T` (=the current turn), but later (in the very same turn `T`!) the game calls `map::process_active_items` which sets `fridge` back to the current turn (`T`).

After 10 turns (or more) `item::rotten` is called again. `since` is now set to `last_rot_check` (=`T`), and `until` is now set to `fridge` (=`T`), which makes both if conditions false and therefor does never increase rot`.

The PR makes the item accumulate rot from `fridge` up until now.