fix panic for stats wrapper

observer/stats/wrapper/conn.go

@@ -152,7 +152,9 @@ func (c *udpConn) SetWriteBuffer(n int) error {
 func (c *udpConn) Read(b []byte) (n int, err error) {
 	if nc, ok := c.PacketConn.(io.Reader); ok {
 		n, err = nc.Read(b)
+		if c.stats != nil {
 			c.stats.Add(stats.KindInputBytes, int64(n))
+		}
 		return
 	}
 	err = errUnsupport
@@ -161,14 +163,18 @@ func (c *udpConn) Read(b []byte) (n int, err error) {
 
 func (c *udpConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
 	n, addr, err = c.PacketConn.ReadFrom(p)
+	if c.stats != nil {
 		c.stats.Add(stats.KindInputBytes, int64(n))
+	}
 	return
 }
 
 func (c *udpConn) ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error) {
 	if nc, ok := c.PacketConn.(udp.ReadUDP); ok {
 		n, addr, err = nc.ReadFromUDP(b)
+		if c.stats != nil {
 			c.stats.Add(stats.KindInputBytes, int64(n))
+		}
 		return
 	}
 	err = errUnsupport
@@ -178,7 +184,9 @@ func (c *udpConn) ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error) {
 func (c *udpConn) ReadMsgUDP(b, oob []byte) (n, oobn, flags int, addr *net.UDPAddr, err error) {
 	if nc, ok := c.PacketConn.(udp.ReadUDP); ok {
 		n, oobn, flags, addr, err = nc.ReadMsgUDP(b, oob)
+		if c.stats != nil {
 			c.stats.Add(stats.KindInputBytes, int64(n))
+		}
 		return
 	}
 	err = errUnsupport
@@ -188,7 +196,9 @@ func (c *udpConn) ReadMsgUDP(b, oob []byte) (n, oobn, flags int, addr *net.UDPAd
 func (c *udpConn) Write(b []byte) (n int, err error) {
 	if nc, ok := c.PacketConn.(io.Writer); ok {
 		n, err = nc.Write(b)
+		if c.stats != nil {
 			c.stats.Add(stats.KindOutputBytes, int64(n))
+		}
 		return
 	}
 	err = errUnsupport
@@ -197,14 +207,18 @@ func (c *udpConn) Write(b []byte) (n int, err error) {
 
 func (c *udpConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
 	n, err = c.PacketConn.WriteTo(p, addr)
+	if c.stats != nil {
 		c.stats.Add(stats.KindOutputBytes, int64(n))
+	}
 	return
 }
 
 func (c *udpConn) WriteToUDP(b []byte, addr *net.UDPAddr) (n int, err error) {
 	if nc, ok := c.PacketConn.(udp.WriteUDP); ok {
 		n, err = nc.WriteToUDP(b, addr)
+		if c.stats != nil {
 			c.stats.Add(stats.KindOutputBytes, int64(n))
+		}
 		return
 	}
 	err = errUnsupport
@@ -214,7 +228,9 @@ func (c *udpConn) WriteToUDP(b []byte, addr *net.UDPAddr) (n int, err error) {
 func (c *udpConn) WriteMsgUDP(b, oob []byte, addr *net.UDPAddr) (n, oobn int, err error) {
 	if nc, ok := c.PacketConn.(udp.WriteUDP); ok {
 		n, oobn, err = nc.WriteMsgUDP(b, oob, addr)
+		if c.stats != nil {
 			c.stats.Add(stats.KindOutputBytes, int64(n))
+		}
 		return
 	}
 	err = errUnsupport