src: Malloc/Calloc size 0 returns non-null pointer

Change `Malloc()/Calloc()` so that size zero does not return a null
pointer, consistent with prior behavior.

Fixes: https://github.com/nodejs/node/issues/8571
PR-URL: https://github.com/nodejs/node/pull/8572


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@keybase.io>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>