repl: eliminate var in function _memory

PR-URL: https://github.com/nodejs/node/pull/26496


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>