Broken find while references are duplicated

Attached failing test shows the problem

```
> map = Blurrily::Map.new
=> #<Blurrily::Map:0x007fefa4a75c60>
> map.put('london', 13)
=> nil
> map.put('londos', 13)
=> nil
> map.find('londi')
Assertion failed: ((int) match_ptr->matches <= nb_trigrams), function blurrily_storage_find, file storage.c, line 485.
Abort trap: 6
```