buffer: safeguard against accidental kNoZeroFill

This makes sure that `kNoZeroFill` flag is not accidentally set by
moving the all the flag operations directly inside `createBuffer()`.
It safeguards against logical errors like
https://github.com/nodejs/node/issues/6006.

This also ensures that `kNoZeroFill` flag is always restored to 0 using
a try-finally block, as it could be not restored to 0 in cases of failed
or zero-size `Uint8Array` allocation.
It safeguards against errors like
https://github.com/nodejs/node/issues/2930.
It also makes the `size > 0` check not needed there.

PR-URL: https://github.com/nodejs/node-private/pull/35


Reviewed-By: James M Snell <jasnell@gmail.com>