string_decoder: fix performance regression

This commit reverts the const usage introduced by 68a6abc8
because v8 currently cannot optimize functions that contain
these uses of const (unsupported phi use of const variable).
The performance difference in this case can be up to ~130%
for non-ascii/binary string encodings.

PR-URL: https://github.com/nodejs/node/pull/5134


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>