Adding the same defensive check

The error seen in ci.jenkins-ci.org is:

+ /usr/local/groovy/bin/groovy ./release.rss.groovy http://updates.jenkins-ci.org/release-history.json
Caught: java.lang.NullPointerException: Cannot invoke method lastIndexOf() on null object
    at release.rss.toGA(release.rss.groovy:21)
    at release.rss$_run_closure2_closure5_closure6_closure7.doCall(release.rss.groovy:46)
    at release.rss$_run_closure2_closure5_closure6_closure7.doCall(release.rss.groovy)
    at release.rss$_run_closure2_closure5_closure6.doCall(release.rss.groovy:45)
    at release.rss$_run_closure2_closure5.doCall(release.rss.groovy:44)
    at release.rss$_run_closure2.doCall(release.rss.groovy:43)
    at release.rss$_run_closure2.doCall(release.rss.groovy)
    at release.rss.run(release.rss.groovy:35)