return an infinite bandwidth if the RTT is zero

internal/congestion/bandwidth.go

@@ -1,6 +1,7 @@
 package congestion
 
 import (
+	"math"
 	"time"
 
 	"github.com/lucas-clemente/quic-go/internal/protocol"
@@ -9,6 +10,8 @@ import (
 // Bandwidth of a connection
 type Bandwidth uint64
 
+const infBandwidth Bandwidth = math.MaxUint64
+
 const (
 	// BitsPerSecond is 1 bit per second
 	BitsPerSecond Bandwidth = 1