Protect websocket against oversized messages.

The maximum message size of 1MB is probably overkill, but SDP
can grow pretty large.  Thanks to Vinayak Mishra.

rtpconn/webclient.go

@@ -832,11 +832,14 @@ func readMessage(conn *websocket.Conn, m *clientMessage) error {
 	return conn.ReadJSON(&m)
 }
 
+const maxWSMessageSize = 1024 * 1024
 const protocolVersion = "2"
 
 func StartClient(conn *websocket.Conn, addr net.Addr) (err error) {
 	var m clientMessage
 
+	conn.SetReadLimit(maxWSMessageSize)
+
 	err = readMessage(conn, &m)
 	if err != nil {
 		conn.Close()