net: avoid nil dereference if /etc/services can't be opened

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4081041

src/pkg/net/port.go

@@ -18,7 +18,9 @@ var onceReadServices sync.Once
 func readServices() {
 	services = make(map[string]map[string]int)
 	var file *file
-	file, servicesError = open("/etc/services")
+	if file, servicesError = open("/etc/services"); servicesError != nil {
+		return
+	}
 	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
 		// "http 80/tcp www www-http # World Wide Web HTTP"
 		if i := byteIndex(line, '#'); i >= 0 {