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>

src/widgets/kernel/qtooltip.cpp

@@ -273,7 +273,7 @@ void QTipLabel::hideTipImmediately()
 
 void QTipLabel::setTipRect(QWidget *w, const QRect &r)
 {
-    if (!rect.isNull() && !w)
+    if (!r.isNull() && !w)
         qWarning("QToolTip::setTipRect: Cannot pass null widget if rect is set");
     else{
         widget = w;