server/tls: allocate tls.Config{} properly. (#2537)

There is a golang bug which crashes the server, allocate tls.Config properly to avoid
this problem.

```
2016/08/22 20:21:03 http: panic serving 127.0.0.1:40744: runtime error: invalid memory address or nil pointer dereference
goroutine 38 [running]:
net/http.(*conn).serve.func1(0xc820526680)
    /home/harsha/.gimme/versions/go1.6.2.linux.amd64/src/net/http/server.go:1389 +0xc1
panic(0xbefa80, 0xc820010140)
    /home/harsha/.gimme/versions/go1.6.2.linux.amd64/src/runtime/panic.go:443 +0x4e9
crypto/tls.(*Conn).serverHandshake(0xc820368300, 0x0, 0x0)
    /home/harsha/.gimme/versions/go1.6.2.linux.amd64/src/crypto/tls/handshake_server.go:43 +0x4d6
crypto/tls.(*Conn).Handshake(0xc820368300, 0x0, 0x0)
    /home/harsha/.gimme/versions/go1.6.2.linux.amd64/src/crypto/tls/conn.go:1035 +0x169
net/http.(*conn).serve(0xc820526680)
    /home/harsha/.gimme/versions/go1.6.2.linux.amd64/src/net/http/server.go:1405 +0x382
created by net/http.(*Server).Serve
    /home/harsha/.gimme/versions/go1.6.2.linux.amd64/src/net/http/server.go:2137 +0x44e
```

Fixes #2536

cmd/server-mux.go

@@ -130,11 +130,8 @@ type MuxListener struct {
 // or invalid
 func NewMuxListener(listener net.Listener, wg *sync.WaitGroup, certPath, keyPath string) (*MuxListener, error) {
 	var err error
-	var config *tls.Config
-	config = nil
-
-	if certPath != "" {
-		config = &tls.Config{}
+	config := &tls.Config{} // Always instantiate.
+	if certPath != "" && keyPath != "" {
 		if config.NextProtos == nil {
 			config.NextProtos = []string{"http/1.1", "h2"}
 		}