buffer: fix backwards incompatibility

4a86803f introduced a backwards incompatibility by accident
and was not caught due to an existing test that wasn't strict enough.

This commit fixes both the backwards incompatibility and the test.

PR-URL: https://github.com/nodejs/node/pull/12439


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>