Bug: U+D908 breaks form

Discovered when compressing the string "Hellloooo!" with lz64:

```
 ҅〶ჶ�䐀
```

[https://lz64.string-248-no.pages.dev/?s=Hellloooo!](https://lz64.string-248-no.pages.dev/?s=Hellloooo!)

This breaks the form when used as input. 

https://unicodeplus.com/U+D908