[EXT] Added more defensive code in the `Test::Cluster#__print_cluster_info` method

Constantly failing on Jenkins...

https://build-eu-1.elasticsearch.org/job/es-ruby_core/ES_V=1.3_nightly,RUBY_V=1.9.3-p448,TEST_TYPE=integration,jdk=JDK7,label=metal-pool/lastBuild/console
Release EXT:0.0.17