Maximum subarray O(n) solution incorrect

The [current implementation](https://mgechev.github.io/javascript-algorithms/searching_maximum-subarray.js.html) will fail the following tests:

```js
maxSubarray([]); // returns 0, should return `undefined`
maxSubarray([-42]); // returns 0, should return 42
maxSubArray([-10, -1, -2, -3, -1]); // returns 0, should return -1
```