[quicwg/base-drafts] Largest Reference algorithm can produce invalid values (#2112)

[Martin Thomson <[e-mail]>]

Feed the algorithm a LargestReference of 1, TotalNumberOfInserts of 2, and MaxEntries of 10.

```python
   if LargestReference > 0:
      LargestReference -= 1  # == 0
      CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries) # == 2

      if CurrentWrapped >= LargestReference + MaxEntries:  # == False
         # Largest Reference wrapped around 1 extra time
         LargestReference += 2 * MaxEntries
      else if CurrentWrapped + MaxEntries < LargestReference:   # == False
         # Decoder wrapped around 1 extra time
         CurrentWrapped += 2 * MaxEntries

      LargestReference += TotalNumberOfInserts - CurrentWrapped # == no change
```

LargestReference is therefore 0, and invalid.

Do the same for LargestReference of 18 and you get something even better: -2.

I think that the underflow check in #1904 fixes this.

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