tmc-check-convert-results crashes with "source sequence is illegal/malformed utf-8"

```
/usr/lib/ruby/2.4.0/json/common.rb:224:in `generate': source sequence is illegal/malformed utf-8 (JSON::GeneratorError)
	from /usr/lib/ruby/2.4.0/json/common.rb:224:in `generate'
	from /usr/bin/tmc-check-convert-results:53:in `<main>'
```
From https://tmc.mooc.fi/submissions/2534753