buffer: refactor redeclared variables

A handful of variable declarations in `lib/buffer.js` redeclare the same
variable in the same scope. This change removes each redeclaration by
switching to `const`, switching to `let`, or explicitly hoisting the
`var` declaration.

PR-URL: https://github.com/nodejs/node/pull/4886


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>