buffer: refactor slowToString

Since slowToString only has one callsite, refactor to eliminate
the use of call.

PR-URL: https://github.com/nodejs/node/pull/11358


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>