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.

This changes does not affect performance.

CI: https://ci.nodejs.org/job/node-test-pull-request/7424/

/cc @trott

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

• buffer