path: fix basename() regressions

This commit fixes a regression in basename() for the case when a
supplied extension name completely matches the resulting basename.

Fixes: https://github.com/nodejs/node/issues/6587
PR-URL: https://github.com/nodejs/node/pull/6590


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>