dgram: use Buffer.alloc(0) for zero-size buffers

There is no difference between alloc(0) and allocUnsafe(0), so there is
no reason to confuse anyone reading the code with an additional call to
allocUnsafe.

PR-URL: https://github.com/nodejs/node/pull/8751


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