app-inspector 增加crash 保护

[SamuelZhaoY]

给 app-inspector 针对"" 以及 {} 类型的特殊source response 加保护

 <-- GET /
The source may be wrong, please report with below message at:
    https://github.com/macacajs/app-inspector/issues/new
    ****** xctest source start *******
    {}
    '****** xctest source end *******

  TypeError: Cannot read property 'type' of undefined
      at adaptor (/usr/local/lib/node_modules/app-inspector/lib/ios.js:23:38)
      at Object.exports.dumpXMLAndScreenShot (/usr/local/lib/node_modules/app-inspector/lib/ios.js:60:22)
      at exports.dumpXMLAndScreenShot.next (<anonymous>)
      at onFulfilled (/usr/local/lib/node_modules/app-inspector/node_modules/co/index.js:65:19)
      at process._tickCallback (internal/process/next_tick.js:109:7)