[INTERNAL] made RosterTracker class thread safe

At some points the roster may null and so methods might throw
an NPE instead of returning an empty list.

Change-Id: I0008cc54ccb13495648b7961ab7a72057ae5ca4e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1447
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>