AbsCaptureTimeExtension does not encode/decode correctly when captureClockOffset is negative

[enobufs]

Your environment.

• Version: v1.8.13

What did you do?

This unit test reveals the issue:

    t.Run("negative captureClockOffset", func(t *testing.T) {
        t0 := time.Now()
        e1 := NewAbsCaptureTimeExtension(t0)
        b1, err1 := e1.Marshal()
        if err1 != nil {
            t.Fatal(err1)
        }
        var o1 AbsCaptureTimeExtension
        if err := o1.Unmarshal(b1); err != nil {
            t.Fatal(err)
        }
        dt1 := o1.CaptureTime().Sub(t0).Seconds()
        if dt1 < -0.001 || dt1 > 0.001 {
            t.Fatalf("timestamp differs, want %v got %v (dt=%f)", t0, o1.CaptureTime(), dt1)
        }
        if o1.EstimatedCaptureClockOffsetDuration() != nil {
            t.Fatalf("duration differs, want nil got %d", o1.EstimatedCaptureClockOffsetDuration())
        }

        e2 := NewAbsCaptureTimeExtensionWithCaptureClockOffset(t0, -1250*time.Millisecond)
        b2, err2 := e2.Marshal()
        if err2 != nil {
            t.Fatal(err2)
        }
        var o2 AbsCaptureTimeExtension
        if err := o2.Unmarshal(b2); err != nil {
            t.Fatal(err)
        }
        dt2 := o1.CaptureTime().Sub(t0).Seconds()
        if dt2 < -0.001 || dt2 > 0.001 {
            t.Fatalf("timestamp differs, want %v got %v (dt=%f)", t0, o2.CaptureTime(), dt2)
        }
        if *o2.EstimatedCaptureClockOffsetDuration() != -1250*time.Millisecond {
            t.Fatalf("duration differs, want -1250ms got %v", *o2.EstimatedCaptureClockOffsetDuration())
        }
    })

What did you expect?

The test pass

I will send a PR shortly.