 Fix: `end_slot` returns incorrect value (#2138)
 Fix: `end_slot` returns incorrect value (#2138)

## Issue Addressed

`Epoch::end_slot()` returns incorrect value when the epoch is the last epoch which can be represented by u64.

```rust
        let slots_per_epoch = 32;

        // The last epoch which can be represented by u64.
        let epoch = Epoch::new(u64::max_value() / slots_per_epoch);

        println!("{}", epoch.end_slot(slots_per_epoch));
       // Slot(18446744073709551614)
       // -> correctly, the result should be `Slot(18446744073709551615)`.
```



sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse