src: only memcmp if length > 0 in Buffer::Compare

Both pointer arguments to memcmp are defined as non-null
and compiler optimizes upon that.

PR-URL: https://github.com/nodejs/node/pull/2544


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: thefourtheye - Sakthipriyan Vairamani <thechargingvolcano@gmail.com>