buffer: speed up base64 encoding by 20%

Remove a lot of branches from the inner loop. Speeds up buf.toString('base64')
by about 20%.

Before:

  $ time out/Release/node benchmark/buffer-base64-encode.js
  real    0m6.607s
  user    0m5.508s
  sys     0m1.088s

After:

  $ time out/Release/node benchmark/buffer-base64-encode.js
  real    0m5.520s
  user    0m4.520s
  sys     0m0.992s