buffers: honor length argument in base64 decoder

Honor the length argument in `buf.write(s, 0, buf.length, 'base64')`. Before
this commit, the length argument was ignored. The decoder would keep writing
until it hit the end of the buffer. Since most buffers in Node are slices of
a parent buffer (the slab), this bug would overwrite the content of adjacent
buffers.

The bug is trivially demonstrated with the following test case:

    var assert = require('assert');
    var a = Buffer(3);
    var b = Buffer('xxx');
    a.write('aaaaaaaa', 'base64');
    assert.equal(b.toString(), 'xxx');

This commit coincidentally also fixes a bug where Buffer._charsWritten was not
updated for zero length buffers.