don't return uninitialized value if there's no backend object: CID 3681
don't return uninitialized value if there's no backend object: CID 3681

svn path=/trunk/KDE/kdelibs/phonon/; revision=679242

sandsmark/phonon-visualization-gsoc,sandsmark/phonon-visualization-gsoc,sandsmark/phonon-visualization-gsoc,sandsmark/phonon-visualization-gsoc