buffer: fix transcode for single-byte enc to ucs2

Fix `buffer.transcode()` for transcoding from single-byte character
encodings to UCS2.

These would previously perform out-of-bounds reads due to an
incorrect `sizeof()` expression.

Fixes: https://github.com/nodejs/node/issues/9834
PR-URL: https://github.com/nodejs/node/pull/9838


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>