net: fix panic in TestDialerDualStack

This change ensures that the test does not try to close dual stack
listeners which have not yet been opened.

Spotted in crash here
http://build.golang.org/log/e5843777df400868ce708b7f00c50dc32c2ec478

Change-Id: I79d513e166effb3e018e2b9dfc23751d92fcbe4b
Reviewed-on: https://go-review.googlesource.com/10371Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/mockserver_test.go

@@ -186,7 +186,7 @@ func newDualStackServer(lns []streamListener) (*dualStackServer, error) {
 	for i := range dss.lns {
 		ln, err := Listen(dss.lns[i].network, JoinHostPort(dss.lns[i].address, dss.port))
 		if err != nil {
-			for _, ln := range dss.lns {
+			for _, ln := range dss.lns[:i] {
 				ln.Listener.Close()
 			}
 			return nil, err