[FIX] #825 Client disappear from roster after leaving the session

Change-Id: Id21c5030f64a00b689d0e1aabf0936a555444fc1
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1390
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>