buffer: do not affect memory after target for utf16 write

Do not write one character too much before shifting the whole result
to the left when using UTF16-LE, possibly overwriting already-used
memory while doing so.

Fixes: https://github.com/nodejs/node/issues/26422

PR-URL: https://github.com/nodejs/node/pull/26432
Fixes: https://github.com/nodejs/node/issues/26422


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>