benchmark: fix buffer-base64-decode.js #27260

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.

Using `len=2` in test-benchmark-buffer was resulting in a `RangeError` in buffer-base64-encode.js. Change to `len=256` which works in all buffer benchmarks.