Panic when allocating to big numbers

When trying to allocate an amount to a big amount net, it results on a panic error.

How to reproduce:

Link: https://play.golang.org/p/lZaEXY0Jaqg

```
package main

import (
	"github.com/Rhymond/go-money"
)

func main() {	
	net := money.New(2911127909321522, "EUR")
	net.Allocate(30000)
}
```

