The way to calculate the average in the BinarySearch may cause overflow.

In BinarySearch, there are a code may cause overflow.

```go
int((maxIndex + minIndex) / 2)
```

If (maxIndex + minIndex) is greater than max int, there occurs overflow and return wrong number.
We should calcurate like below.

```go
int(minIndex + (maxIndex-minIndex)/2)
```

I'll make a PR.