playback: fix panic when MP4 muxer flushes with no samples (#5867)

Co-authored-by: Arturo2511 <arturomozzon@gmail.com>

internal/playback/muxer_mp4.go

@@ -90,7 +90,7 @@ func (w *muxerMP4) writeFinalDTS(dts int64) {
 }
 
 func (w *muxerMP4) flush() error {
-	if len(w.curTrack.Samples) == 0 || w.curTrack.lastDTS < 0 {
+	if w.curTrack == nil || len(w.curTrack.Samples) == 0 || w.curTrack.lastDTS < 0 {
 		return recordstore.ErrNoSegmentsFound
 	}
 

internal/playback/muxer_mp4_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/bluenviron/mediacommon/v2/pkg/formats/fmp4"
 	mcodecs "github.com/bluenviron/mediacommon/v2/pkg/formats/mp4/codecs"
+	"github.com/bluenviron/mediamtx/internal/recordstore"
 	"github.com/bluenviron/mediamtx/internal/test"
 	"github.com/stretchr/testify/require"
 )
@@ -57,3 +58,30 @@ func TestMuxerMP4EmptyTracks(t *testing.T) {
 
 	require.Greater(t, buf.Len(), 0)
 }
+
+// Test that flushing without any written sample returns an error instead of panicking
+func TestMuxerMP4NoSamples(t *testing.T) {
+	var buf bytes.Buffer
+
+	mux := &muxerMP4{
+		w: &buf,
+	}
+
+	init := &fmp4.Init{
+		Tracks: []*fmp4.InitTrack{
+			{
+				ID:        1,
+				TimeScale: 90000,
+				Codec: &mcodecs.H264{
+					SPS: test.FormatH264.SPS,
+					PPS: test.FormatH264.PPS,
+				},
+			},
+		},
+	}
+
+	mux.writeInit(init)
+
+	err := mux.flush()
+	require.ErrorIs(t, err, recordstore.ErrNoSegmentsFound)
+}