buffer: ignore negative allocation lengths

Treat negative length arguments to `Buffer()`/`allocUnsafe()`
as if they were zero so the allocation does not affect the
pool’s offset.

Fixes: https://github.com/nodejs/node/issues/7047

Refs: https://github.com/nodejs/node/pull/7051
PR-URL: https://github.com/nodejs/node/pull/7221


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>