QtCompositor: fix null pointer reference

Don't call a method on a null pointer in QWaylandInputPanel

Change-Id: I1e32a84323a06267bd5e3a4aee6c0a94c98b8df3
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>