[KARAF-2577] Prevent NPE when no CommandSession exists

git-svn-id: https://svn.apache.org/repos/asf/karaf/trunk@1541216 13f79535-47bb-0310-9956-ffa450edef68
[KARAF-2575] Add missing commands in bin/instance script

git-svn-id: https://svn.apache.org/repos/asf/karaf/trunk@1541217 13f79535-47bb-0310-9956-ffa450edef68