src: fix bug in MallocedBuffer constructor

This should be a generic type even though we are
currently only using `char` as `T`.

PR-URL: https://github.com/nodejs/node/pull/23434


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>