generateThumbnail produces toxic image

### How to reproduce these conditions

I'm using the sample code as is: It builds and uploads and runs.
The thumb_ image can't be downloaded and just 'hangs' in the control panel.
I've tried .jpg and .png, small and large. All produce the same failure.
Logs say it ran 'ok'. About 6 seconds for a 1 meg image.

Running latest software:

```
"engines": {
"node": "10"
},
"dependencies": {
"child-process-promise": "^2.2.1",
"firebase-admin": "^8.6.0",
"firebase-functions": "^3.3.0"
}
```

firebase-tools are current.

Here is what the console looks like after an upload. The original image uploads fine.
The generated thumb never presents an image or a link (though its name and size are correct)