fix transport header error with some Hikvision cameras

fixes 'invalid transport header: encoding/hex: invalid byte: U+0020 ' ''.

pkg/headers/transport.go

@@ -184,6 +184,17 @@ func (h *Transport) Read(v base.HeaderValue) error {
 			h.ServerPorts = ports
 
 		case "ssrc":
+			// replace initial spaces
+			var b strings.Builder
+			b.Grow(len(v))
+			i := 0
+			for ; i < len(v) && v[i] == ' '; i++ {
+				b.WriteByte('0')
+			}
+			b.WriteString(v[i:])
+			v = b.String()
+
+			// add initial zeros
 			if (len(v) % 2) != 0 {
 				v = "0" + v
 			}

pkg/headers/transport_test.go

@@ -155,6 +155,28 @@ var casesTransport = []struct {
 			}(),
 		},
 	},
+	{
+		"hikvision ssrc with initial spaces",
+		base.HeaderValue{`RTP/AVP/UDP;unicast;client_port=14186;server_port=8052;ssrc= 4317f;mode=play`},
+		base.HeaderValue{`RTP/AVP;unicast;client_port=14186-14187;server_port=8052-8053;ssrc=0004317F;mode=play`},
+		Transport{
+			Protocol: TransportProtocolUDP,
+			Delivery: func() *TransportDelivery {
+				v := TransportDeliveryUnicast
+				return &v
+			}(),
+			Mode: func() *TransportMode {
+				v := TransportModePlay
+				return &v
+			}(),
+			ClientPorts: &[2]int{14186, 14187},
+			ServerPorts: &[2]int{8052, 8053},
+			SSRC: func() *uint32 {
+				v := uint32(0x04317f)
+				return &v
+			}(),
+		},
+	},
 }
 
 func TestTransportRead(t *testing.T) {