hls muxer: fix crash (#1394)

this happened when variant is lowLatency and two AUs have adjacent
DTSs, resulting in a sample duration equal to zero.

discovered while investigating #1156

internal/hls/muxer_test.go

@@ -566,3 +566,30 @@ func TestMuxerDoubleRead(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, byts1, byts2)
 }
+
+func TestMuxerFMP4ZeroDuration(t *testing.T) {
+	videoTrack := &format.H264{
+		PayloadTyp:        96,
+		SPS:               testSPS,
+		PPS:               []byte{0x08},
+		PacketizationMode: 1,
+	}
+
+	m, err := NewMuxer(MuxerVariantLowLatency, 3, 1*time.Second, 0, 50*1024*1024, videoTrack, nil)
+	require.NoError(t, err)
+	defer m.Close()
+
+	err = m.WriteH26x(time.Now(), 0, [][]byte{
+		testSPS, // SPS
+		{8},     // PPS
+		{5},     // IDR
+	})
+	require.NoError(t, err)
+
+	err = m.WriteH26x(time.Now(), 1*time.Nanosecond, [][]byte{
+		testSPS, // SPS
+		{8},     // PPS
+		{5},     // IDR
+	})
+	require.NoError(t, err)
+}

internal/hls/muxer_variant_fmp4_segmenter.go

@@ -25,21 +25,22 @@ func partDurationIsCompatible(partDuration time.Duration, sampleDuration time.Du
 	return partDuration > ((f * 85) / 100)
 }
 
+func partDurationIsCompatibleWithAll(partDuration time.Duration, sampleDurations map[time.Duration]struct{}) bool {
+	for sd := range sampleDurations {
+		if !partDurationIsCompatible(partDuration, sd) {
+			return false
+		}
+	}
+	return true
+}
+
 func findCompatiblePartDuration(
 	minPartDuration time.Duration,
 	sampleDurations map[time.Duration]struct{},
 ) time.Duration {
 	i := minPartDuration
 	for ; i < 5*time.Second; i += 5 * time.Millisecond {
-		isCompatible := func() bool {
-			for sd := range sampleDurations {
-				if !partDurationIsCompatible(i, sd) {
-					return false
-				}
-			}
-			return true
-		}()
-		if isCompatible {
+		if partDurationIsCompatibleWithAll(i, sampleDurations) {
 			break
 		}
 	}
@@ -147,6 +148,11 @@ func (m *muxerVariantFMP4Segmenter) adjustPartDuration(du time.Duration) {
 		return
 	}
 
+	// avoid a crash by skipping invalid durations
+	if du == 0 {
+		return
+	}
+
 	if _, ok := m.sampleDurations[du]; !ok {
 		m.sampleDurations[du] = struct{}{}
 		m.adjustedPartDuration = findCompatiblePartDuration(