Execa function may not pass stdout in callback function if run it wit…

…h the `{ stdio: 'inherit' }` options.

Added a check for the presence of stdout in the object passed to the execa callback function.

lib/youtube-dl.js

@@ -22,7 +22,7 @@ const execa = universalify.fromPromise(require('execa'))
 function youtubeDl (args, options, cb) {
  return execa(ytdlBinary, args, options, function done (err, output) {
  if (err) return cb(err)
- return cb(null, output.stdout.trim().split(/\r?\n/))
+ return cb(null, output.stdout ? output.stdout.trim().split(/\r?\n/) : undefined)
  })
 }