src: make base64 decoding 50% faster

Make the inner loop execute fewer compare-and-branch executions per
processed byte, resulting in a 50% or more speedup.

This coincidentally fixes an out-of-bounds read:

    while (unbase64(*src) < 0 && src < srcEnd)

Should have read:

    while (src < srcEnd && unbase64(*src) < 0)

But this commit removes the offending code altogether.

Fixes: https://github.com/nodejs/io.js/issues/2166
PR-URL: https://github.com/nodejs/io.js/pull/2193


Reviewed-By: Trevor Norris <trev.norris@gmail.com>