Fix empty twcc packet unmarshalling

Currently empty twcc packets fail unmarshalling because they are too
short. These packets aren't actually invalid, however.

Author: kevmo314

AUTHORS.txt

@@ -12,6 +12,7 @@ Gabor Pongracz <gabor.pongracz@proemergotech.com>
 Hugo Arregui <hugo.arregui@gmail.com>
 Hugo Arregui <hugo@decentraland.org>
 Juliusz Chroboczek <jch@irif.fr>
+Kevin Wang <kevmo314@gmail.com>
 lllf <littlelightlittlefire@gmail.com>
 Luke Curley <kixelated@gmail.com>
 Max Hawkins <maxhawkins@gmail.com>

transport_layer_cc.go

@@ -447,7 +447,7 @@ func (t *TransportLayerCC) Unmarshal(rawPacket []byte) error { //nolint:gocognit
 	// header's length + payload's length
 	totalLength := 4 * (t.Header.Length + 1)
 
-	if totalLength <= headerLength+packetChunkOffset {
+	if totalLength < headerLength+packetChunkOffset {
 		return errPacketTooShort
 	}
 

transport_layer_cc_test.go

@@ -630,6 +630,31 @@ func TestTransportLayerCC_Unmarshal(t *testing.T) {
 			},
 			WantError: nil,
 		},
+		{
+			Name: "example3",
+			Data: []byte{
+				0x8f, 0xcd, 0x0, 0x4,
+				0x9a, 0xcb, 0x4, 0x42,
+				0x0, 0x0, 0x0, 0x0,
+				0x0, 0x0, 0x0, 0x0,
+				0x0, 0x0, 0x0, 0x0,
+			},
+			Want: TransportLayerCC{
+				Header: Header{
+					Padding: false,
+					Count:   FormatTCC,
+					Type:    TypeTransportSpecificFeedback,
+					Length:  4,
+				},
+				SenderSSRC:         2596996162,
+				MediaSSRC:          0,
+				BaseSequenceNumber: 0,
+				PacketStatusCount:  0,
+				ReferenceTime:      0,
+				FbPktCount:         0,
+			},
+			WantError: nil,
+		},
 	} {
 		test := test
 		t.Run(test.Name, func(t *testing.T) {