test: add assertions to zero length buffer test

1) Add missing assertion that slicing a 0 length buffer does not throw
2) Add assertion that slicing a 0 length buffer has a length of 0

PR-URL: https://github.com/nodejs/node/pull/8729


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>