`ctx.getImageData` reports `Floating point exception: 8` for canvas with 0 width/height

## Issue

I'm using the latest version `2.0.0-alpha3`, and I keep running into `Floating point exception: 8`

I tried different images, but I still got the same result

## Steps to Reproduce
```js
fs.readFile(__dirname + '/static/t1.jpeg', (err, squid) => {
if (err) throw err;
const img = new Image;
const canvas = createCanvas(img.width, img.height);
const ctx = canvas.getContext('2d');
img.src = squid;
ctx.drawImage(img, 0, 0, img.width, img.height); //this is all good
const imageData = ctx.getImageData(0, 0, img.width, img.height); // return floating point exception: 8
});
```

## Your Environment
* Version of node-canvas (e.g. 1.4.0): 2.0.0-alpha3
* Environment (e.g. node 4.2.0 on Mac OS X 10.8): Mac OS X 10.12.6