serve dnla: fix crash on graceful exit

Before this change, closing a uninitialised chan would cause a crash.

cmd/serve/dlna/dlna.go

@@ -129,9 +129,8 @@ func newServer(f fs.Fs, opt *dlnaflags.Options) (*server, error) {
 		FriendlyName:     friendlyName,
 		RootDeviceUUID:   makeDeviceUUID(friendlyName),
 		Interfaces:       interfaces,
-
+		waitChan:         make(chan struct{}),
 		httpListenAddr:   opt.ListenAddr,
-
 		f:                f,
 		vfs:              vfs.New(f, &vfsflags.Opt),
 	}