net: fix ParseIP

Fixes #1695.

R=golang-dev, rsc
CC=golang-dev, r
https://golang.org/cl/4418042

src/pkg/net/ip.go

@@ -436,7 +436,7 @@ func parseIPv6(s string) IP {
 		}
 
 		// Otherwise must be followed by colon and more.
-		if s[i] != ':' && i+1 == len(s) {
+		if s[i] != ':' || i+1 == len(s) {
 			return nil
 		}
 		i++

src/pkg/net/ip_test.go

@@ -29,6 +29,7 @@ var parseiptests = []struct {
 	{"127.0.0.1", IPv4(127, 0, 0, 1)},
 	{"127.0.0.256", nil},
 	{"abc", nil},
+	{"123:", nil},
 	{"::ffff:127.0.0.1", IPv4(127, 0, 0, 1)},
 	{"2001:4860:0:2001::68",
 		IP{0x20, 0x01, 0x48, 0x60, 0, 0, 0x20, 0x01,