repl: fix /dev/null history file regression

This fixes a regression from bb041ea5 where ftruncate() fails on a file symlinked to /dev/null.

/cc @bzoz @jasnell

CI: https://ci.nodejs.org/job/node-test-pull-request/7761/

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)

• repl