buffer: don't CHECK on zero-sized realloc

malloc(0) and realloc(ptr, 0) have implementation-defined behavior in
that the standard allows them to either return a unique pointer or a
nullptr for zero-sized allocation requests.  Normalize by always using
a nullptr.

Fixes: https://github.com/nodejs/node/issues/3496
PR-URL: https://github.com/nodejs/node/pull/3499


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