src: add Malloc() size param + overflow detection

Adds an optional second parameter to `node::Malloc()` and
an optional third parameter to `node::Realloc()` giving the
size/number of items to be allocated, in the style of `calloc(3)`.

Use a proper overflow check using division;
the previous `CHECK_GE(n * size, n);` would not detect all cases
of overflow (e.g. `size == SIZE_MAX / 2 && n == 3`).

PR-URL: https://github.com/nodejs/node/pull/8482


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>