Fix ArithmeticException crash

ArithmeticException is caused when native currency is JPY.

How to reproduce the crash:

1. Set Native currency to JPY.
2. Enable "Don't keep activity" in developer settings.
3. Open Coinbase.
4. Back to home screen.
5. Open Coinbase Android again.

Smallest problem code is:
```java
Money.of(CurrencyUnit.JPY, 0);
// causes ArithmeticException
```