net: ParseIP should return nil if :: doesn't expand in an IPv6 address.

Per RFC 4291, 'The use of "::" indicates one or more groups of 16 bits of zeros.'
Fixes #6628

R=golang-dev, rsc, minux.ma, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/15990043

src/pkg/net/ip.go

@@ -623,6 +623,9 @@ func parseIPv6(s string, zoneAllowed bool) (ip IP, zone string) {
 		for k := ellipsis + n - 1; k >= ellipsis; k-- {
 			ip[k] = 0
 		}
+	} else if ellipsis >= 0 {
+		// Ellipsis must represent at least one 0 group.
+		return nil, zone
 	}
 	return ip, zone
 }

src/pkg/net/ip_test.go

@@ -25,6 +25,7 @@ var parseIPTests = []struct {
 	{"fe80::1%lo0", nil},
 	{"fe80::1%911", nil},
 	{"", nil},
+	{"a1:a2:a3:a4::b1:b2:b3:b4", nil}, // Issue 6628
 }
 
 func TestParseIP(t *testing.T) {