stop null referencing error

[dpjanes]

I'm constantly getting the following crash

…node_modules/castv2-client/node_modules/castv2/lib/client.js:53
self.ps.removeListener('packet', onpacket);
       ^
TypeError: Cannot read property 'removeListener' of null
at TLSSocket.onclose (/Users/davidjanes/iotdb/iot/homestar-chromecast/node_modules/chromecast-js/node_modules/castv2-client/node_modules/castv2/lib/client.js:53:12)
at TLSSocket.g (events.js:199:16)
at TLSSocket.emit (events.js:107:17)
at TCP.close (net.js:485:12)

I'm not sure of the exact triggering mechanism - perhaps onclose is being called twice. It happens after several hours of operation.

The code change checks to see if values are null