buffer: do proper error propagation in addon methods

- Always fulfill the `MaybeLocal<>` contract by scheduling an
  exception when returning an empty value. This was previously
  inconsistent, with no way to know whether an exception was
  be scheduled or not in case of failure.
- Make sure that memory is released exactly once in case of
  failure. Previously, some exit conditions would have leaked
  memory or attempted to free it multiple times.

This should not really affect how `Buffer`s are created by
addons in practice, due to the low frequency with which
these errors would typically occur.

PR-URL: https://github.com/nodejs/node/pull/23939


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>