buffer: fix transcode for single-byte enc to ucs2

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

buffer

Description of change

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

/cc @nodejs/buffer