link references are wrongly parsed inside fenced code blocks

[jgm]

See http://johnmacfarlane.net/babelmark2/?normalize=1&text=%5Bfoo%5D%0A%0A%60%60%60%0A%5Bfoo%5D%3A+%2Furl%0A%60%60%60%0A

Here's the example:

[foo]

```
[foo]: /url
```

[mildsunrise]

👍 yeah, I confirm.