benchmark: fix buffer-base64-decode.js

693401d0 added stricter range checking
for buffer operations and that apparently seems to have uncovered the
fact that one of our benchmarks was overflowing a buffer. Increase the
buffer size so the benchmark doesn't throw an error anymore.

PR-URL: https://github.com/nodejs/node/pull/27260


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>