Fix warning message when re-setting text on tooltip

Fix warning message that gets printed when changing tooltip message
without rect and widget parameters for already shown tooltip widget.

Task-number: QTBUG-39550
Change-Id: I69ed8747e5e77ca618525d74479c72e02b7ab897
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>