fix undefined desc if name = lchown

I have got an error when I try to use koa-sendfile:

/home/fp/tests/work-base/node_modules/koa-sendfile/node_modules/mz/_promisify_all.js:24
  if (desc.get.name === 'deprecated') return true
          ^
TypeError: Cannot read property 'get' of undefined
    at deprecated (/home/fp/tests/work-base/node_modules/koa-sendfile/node_modules/mz/_promisify_all.js:24:11)
    at /home/fp/tests/work-base/node_modules/koa-sendfile/node_modules/mz/_promisify_all.js:6:9
    at Array.forEach (native)
    at module.exports (/home/fp/tests/work-base/node_modules/koa-sendfile/node_modules/mz/_promisify_all.js:5:11)
    at Object.<anonymous> (/home/fp/tests/work-base/node_modules/koa-sendfile/node_modules/mz/fs.js:4:31)
    at Module._compile (module.js:449:26)

The name which generated the error was "lchown".