buffer: fix needle length misestimation for UCS2

Use `StringBytes::Size` to determine the needle string length
instead of assuming latin-1 or UTF-8.

Previously, `Buffer.indexOf` could fail with an assertion failure
when the needle's byte length, but not its character count,
exceeded the haystack's byte length.

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>