fix FIPS regression in 4.x

Checklist

Affected core subsystem(s)

tiny change to lib/_tls_wrap.js

Description of change

This fixes a regression introduced in 4.2.4 where under certain circumstances (I believe mocha test framework, but didn't isolate) the code in getDefaultSessionIdContext() would throw an exception because process.config.variables was undefined and thus doesn't have attributes to access.

Why the project's libs in use were mucking about in process.config like that we'll never know, but this defensive patch makes it work again (like in 4.2.3).

https://github.com/nodejs/node/pull/3755#issuecomment-207066246