buffer: fix lastIndexOf crash for overlong needle

Return -1 in `Buffer.lastIndexOf` if the needle is longer than the
haystack. The previous check only tested the corresponding
condition for forward searches.

This applies only to Node.js v6, as `lastIndexOf` was added in it.

Fixes: https://github.com/nodejs/node/issues/6510
PR-URL: https://github.com/nodejs/node/pull/6511


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>