tools: do not lint commit message if var undefined

Check that $TRAVIS_COMMIT_RANGE is set before trying to lint commit
messages in Travis CI.

Refs: https://github.com/nodejs/node/pull/23572#issuecomment-430850382
Refs: https://github.com/nodejs/node/pull/22842#issuecomment-430850049

PR-URL: https://github.com/nodejs/node/pull/23725


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>