hls: use audioAUCount only if audio is primary track

internal/hls/muxer.go

@@ -158,8 +158,10 @@ func (m *Muxer) WriteAAC(pts time.Duration, aus [][]byte) error {
 			return err
 		}
 
+		if m.videoTrack == nil {
 			m.audioAUCount++
 		}
+	}
 
 	return nil
 }