buffer: fix not return on error

Throwing a JS error from C++ does not mean the function will return
early. This must be done manually.

Also remove extraneous comment no longer relevant.

Fix: 2903030 "buffer: switch API to return MaybeLocal<T>"
PR-URL: https://github.com/nodejs/io.js/pull/2225


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>