stream: refactor redeclared variables

`lib/_stream_readable.js` contained three instances of `var`
declarations occurring twice in the same scope. Refactored to `const` or
`let` as appropriate.

PR-URL: https://github.com/nodejs/node/pull/4816


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>