napi: supress invalid coverity leak message

Coverity was complaining that finalizer was being
leaked in this method, however it should be
freed when the buffer is finalized so I believe
the message is invalid.

Add the required comments to suppress the warning.

PR-URL: https://github.com/nodejs/node/pull/12192


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Hitesh Kanwathirtha <hiteshk@microsoft.com>