delete playlist crash server

```
ok command playlistCreate { id: '2s0HABy3atVEqDOHT-grhxNXhGhU7pD-', name: 'aou' }
ok command playlistRemoveItems {}

/home/andy/dev/groovebasin/lib/player_server.js:1730
trackKey: playlistForEvent.items[itemKey].key,
^
TypeError: Cannot read property 'items' of undefined
at PlayerServer.playlistRemoveItems (/home/andy/dev/groovebasin/lib/player_server.js:1730:31)
at Object.actions.playlistRemoveItems.fn (/home/andy/dev/groovebasin/lib/player_server.js:379:12)
at WebSocketApiClient.onMessage (/home/andy/dev/groovebasin/lib/player_server.js:1047:12)
at WebSocketApiClient.emit (events.js:98:17)
at WebSocketClient. (/home/andy/dev/groovebasin/lib/web_socket_api_client.js:36:10)
at WebSocketClient.emit (events.js:95:17)
at WebSocketClient._transform (/home/andy/dev/groovebasin/node_modules/yawl/index.js:544:16)
at WebSocketClient.Transform._read (_stream_transform.js:179:10)
at WebSocketClient.Transform._write (_stream_transform.js:167:12)
```