qcam: Check that camera can generate configuration from roles

If the camera can not generate a configuration from the requested roles
it returns a nullptr which leads to a nullptr dereference. Fix this by
adding a check that the camera generated a configuration before trying
to access it.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/qcam/main_window.cpp

@@ -296,6 +296,10 @@ int MainWindow::startCapture()
 
 	/* Configure the camera. */
 	config_ = camera_->generateConfiguration(roles);
+	if (!config_) {
+		qWarning() << "Failed to generate configuration from roles";
+		return -EINVAL;
+	}
 
 	StreamConfiguration &cfg = config_->at(0);