buffer: little improve for Buffer.concat method

When buffer list less than 2, no need to calculate the length.
The change's benchmark result is here:
https://gist.github.com/JacksonTian/2c9e2bdec00018e010e6
It improve 15% ~ 25% speed when list only have one buffer,
to other cases no effect.

PR-URL: https://github.com/iojs/io.js/pull/1437


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>