fix support for LPCM tracks with no explicit channel count

(https://github.com/aler9/rtsp-simple-server/issues/1292)

track_test.go

@@ -95,6 +95,28 @@ func TestTrackNewFromMediaDescription(t *testing.T) {
 				ChannelCount: 2,
 			},
 		},
+		{
+			"lpcm 16 with no explicit channel",
+			&psdp.MediaDescription{
+				MediaName: psdp.MediaName{
+					Media:   "audio",
+					Protos:  []string{"RTP", "AVP"},
+					Formats: []string{"97"},
+				},
+				Attributes: []psdp.Attribute{
+					{
+						Key:   "rtpmap",
+						Value: "97 L16/16000",
+					},
+				},
+			},
+			&TrackLPCM{
+				PayloadType:  97,
+				BitDepth:     16,
+				SampleRate:   16000,
+				ChannelCount: 1,
+			},
+		},
 		{
 			"lpcm 24",
 			&psdp.MediaDescription{