make it possible to parse a varint at the end of a reader (#3428)

An io.Reader can read into the buffer and return the io.EOF in the same
call. In fact, that's how the quic.Stream is implemented.
In that case, we shouldn't return an error from quicvarint.Read,
otherwise the caller won't be able to parse a varint sent just before a
stream was closed.

quicvarint/io.go

@@ -31,7 +31,10 @@ func NewReader(r io.Reader) Reader {
 
 func (r *byteReader) ReadByte() (byte, error) {
 	var b [1]byte
-	_, err := r.Reader.Read(b[:])
+	n, err := r.Reader.Read(b[:])
+	if n == 1 && err == io.EOF {
+		err = nil
+	}
 	return b[0], err
 }