net: add nil checks to several String methods to avoid panics

Fixes #945.

R=r
CC=golang-dev
https://golang.org/cl/1848049

src/pkg/net/dnsclient.go

@@ -30,6 +30,9 @@ type DNSError struct {
 }
 
 func (e *DNSError) String() string {
+	if e == nil {
+		return "<nil>"
+	}
 	s := "lookup " + e.Name
 	if e.Server != "" {
 		s += " on " + e.Server

src/pkg/net/iprawsock.go

@@ -30,7 +30,12 @@ type IPAddr struct {
 // Network returns the address's network name, "ip".
 func (a *IPAddr) Network() string { return "ip" }
 
-func (a *IPAddr) String() string { return a.IP.String() }
+func (a *IPAddr) String() string {
+	if a == nil {
+		return "<nil>"
+	}
+	return a.IP.String()
+}
 
 func (a *IPAddr) family() int {
 	if a == nil || len(a.IP) <= 4 {

src/pkg/net/net.go

@@ -129,6 +129,9 @@ type OpError struct {
 }
 
 func (e *OpError) String() string {
+	if e == nil {
+		return "<nil>"
+	}
 	s := e.Op
 	if e.Net != "" {
 		s += " " + e.Net
@@ -164,6 +167,9 @@ type AddrError struct {
 }
 
 func (e *AddrError) String() string {
+	if e == nil {
+		return "<nil>"
+	}
 	s := e.Error
 	if e.Addr != "" {
 		s += " " + e.Addr

src/pkg/net/tcpsock.go

@@ -30,7 +30,12 @@ type TCPAddr struct {
 // Network returns the address's network name, "tcp".
 func (a *TCPAddr) Network() string { return "tcp" }
 
-func (a *TCPAddr) String() string { return joinHostPort(a.IP.String(), itoa(a.Port)) }
+func (a *TCPAddr) String() string {
+	if a == nil {
+		return "<nil>"
+	}
+	return joinHostPort(a.IP.String(), itoa(a.Port))
+}
 
 func (a *TCPAddr) family() int {
 	if a == nil || len(a.IP) <= 4 {

src/pkg/net/udpsock.go

@@ -30,7 +30,12 @@ type UDPAddr struct {
 // Network returns the address's network name, "udp".
 func (a *UDPAddr) Network() string { return "udp" }
 
-func (a *UDPAddr) String() string { return joinHostPort(a.IP.String(), itoa(a.Port)) }
+func (a *UDPAddr) String() string {
+	if a == nil {
+		return "<nil>"
+	}
+	return joinHostPort(a.IP.String(), itoa(a.Port))
+}
 
 func (a *UDPAddr) family() int {
 	if a == nil || len(a.IP) <= 4 {