bench: fix buffers/buffer-base64-encode benchmark

The test is supposed to measure the performance of the base64 encoder
so move the Buffer#write() calls out of the benchmark section.

The overhead of the calls isn't terrible (about 1-3%) but having
unrelated activity in a micro-benchmark is never a good idea.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>