fixes https://github.com/ssbc/patchwork/issues/563
fixes https://github.com/ssbc/patchwork/issues/563

```
TypeError: Cannot read property 'link' of null
    at compareImage (/home/dinosaur/repos/ssbc/patchwork/modules/gathering/sheet/edit.js:159:32)
    at HTMLButtonElement.save (/home/dinosaur/repos/ssbc/patchwork/modules/gathering/sheet/edit.js:112:14)
```

ssbc/patchwork,ssbc/patchwork