benchmark: fix buffer-base64-decode.js

693401d 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: nodejs#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>

benchmark/buffers/buffer-base64-decode.js

@@ -9,11 +9,12 @@ const bench = common.createBenchmark(main, {
 
 function main({ n, size }) {
   const s = 'abcd'.repeat(size);
+  const encodedSize = s.length * 3 / 4;
   // eslint-disable-next-line node-core/no-unescaped-regexp-dot
   s.match(/./);  // Flatten string.
   assert.strictEqual(s.length % 4, 0);
-  const b = Buffer.allocUnsafe(s.length / 4 * 3);
-  b.write(s, 0, s.length, 'base64');
+  const b = Buffer.allocUnsafe(encodedSize);
+  b.write(s, 0, encodedSize, 'base64');
   bench.start();
   for (var i = 0; i < n; i += 1) b.base64Write(s, 0, s.length);
   bench.end(n);