FileUtils: Add some safety checks to removeRecursively().

Change-Id: I657f8416081ce7d253f4a3ef9722a39ac34b5f24
Reviewed-on: http://codereview.qt.nokia.com/1955


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>