Remote peer can panic quinn-proto with zero-length STREAM frames during unordered reads

Summary

When a stream is read in unordered mode (for example through `RecvStream::read(false)` in quinn-proto or `read_unordered` in `quinn`), quinn-proto proactively deduplicates received data by recording received ranges in a `RangeSet` through its `replace` method. When an incoming STREAM frame carried an empty payload, the `Drop` implementation of the iterator returned by `replace` inserted an empty range into the set, violating the invariant that stored ranges are non-empty. Subsequent frames overlapping that region then caused a panic while processing the stream.

A remote peer can trigger this with five small STREAM frames (two of them zero-length) once the application has put the stream in unordered-read mode. The default ordered read path is not affected.

The flaw was corrected in 0.11.18 by ignoring empty ranges in `RangeSet::replace`.

Thanks to Otto Hermann ([@ojhermann](https://github.com/ojhermann)) for reporting this issue.