net: fix Plan 9 build

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/5631051

src/pkg/net/ipsock_plan9.go

@@ -23,7 +23,7 @@ func probeIPv6Stack() (supportsIPv6, supportsIPv4map bool) {
 // parsePlan9Addr parses address of the form [ip!]port (e.g. 127.0.0.1!80).
 func parsePlan9Addr(s string) (ip IP, iport int, err error) {
 	addr := IPv4zero // address contains port only
-	i = byteIndex(s, '!')
+	i := byteIndex(s, '!')
 	if i >= 0 {
 		addr = ParseIP(s[:i])
 		if addr == nil {