Update protobuf.js

https://github.com/NetEase/pomelo/blob/master/lib/components/protobuf.js#L25
https://github.com/NetEase/pomelo/blob/master/lib/components/protobuf.js#L26
這兩行有bug 
this.serverProtosPath和this.clientProtosPath永遠不會是我傳進去的opts.serverProtos
望修正
看來是重構這塊的時候忽略了三項表達式和||的先後順序
Merge pull request #485 from jiangzhuo/patch-1

Update protobuf.js