src: reduce duplicate code in SafeGetenv()

PR-URL: https://github.com/nodejs/node/pull/13220


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>