Return 0 in QtGrabber::grabFrame() if error occurs in QtGrabber::setupDisplay()

Signed-off-by: Paulchen-Panther <Paulchen-Panter@protonmail.com>

libsrc/grabber/qt/QtGrabber.cpp

@@ -96,6 +96,7 @@ void QtGrabber::geometryChanged(const QRect &geo)
 
 int QtGrabber::grabFrame(Image<ColorRgb> & image)
 {
+	if (!_enabled) return 0;
 	if(_screen == nullptr)
 	{
 		// reinit, this will disable capture on failure