[FIX] RoundedComposite did not handle null on setBackground()

The composite will now handle null. In addition it will no longer
dispose a system color which had occurred in some rare
circumstances.

Change-Id: I6fb3139acfcb688e539218206910efe71977b469
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1761
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>