fix NPE (这个词不是太准确，但我想大家懂的）

… … /TransAuto/auto.js:34
              items = result.rss.channel[0].item;
                                ^
TypeError: Cannot read property 'channel' of undefined
    at /home/solocompany/TransAuto/auto.js:34:33
    at Parser.<anonymous>
(/home/solocompany/node_modules/xml2js/lib/xml2js.js:344:20)
    at Parser.EventEmitter.emit (events.js:95:17)
    at Object.saxParser.onclosetag
(/home/solocompany/node_modules/xml2js/lib/xml2js.js:314:24)
    at emit
(/home/solocompany/node_modules/xml2js/node_modules/sax/lib/sax.js:602:3
3)
    at emitNode
(/home/solocompany/node_modules/xml2js/node_modules/sax/lib/sax.js:607:3
)
    at closeTag
(/home/solocompany/node_modules/xml2js/node_modules/sax/lib/sax.js:848:5
)
    at Object.write
(/home/solocompany/node_modules/xml2js/node_modules/sax/lib/sax.js:1267:
29)
    at Parser.exports.Parser.Parser.parseString
(/home/solocompany/node_modules/xml2js/lib/xml2js.js:362:29)
    at Parser.parseString
(/home/solocompany/node_modules/xml2js/lib/xml2js.js:6:61)