Assertion failure when drawing after calling canvas.toBuffer() for PDF or SVG canvas

## Steps to Reproduce

```js
const canvas = createCanvas(200,200,"svg")
const ctx = canvas.getContext("2d");
canvas.toBuffer()
ctx.fillRect(10,10,100,100)
canvas.toBuffer()
```
results in
```
File: cairo-surface.c, Line 385

Expression: ! surface->finished
```

`toBuffer` has a call to `cairo_surface_finish`. *After calling cairo_surface_finish() the only valid operations on a surface are getting and setting user, referencing and destroying, and flushing and finishing it.*

We should at least catch that failure so the process doesn't crash. I don't see a way to actually let the canvas continue to be usable for drawing though. (Thought there might be an easy way to copy the content, but not seeing one.)

## Your Environment
* Version of node-canvas (e.g. 1.4.0): master @ b10d204
* Environment (e.g. node 4.2.0 on Mac OS X 10.8): node 8.x, Win10 x64