qcam: Don't crash if camera can't be opened

If the camera specified on the command line can't be opened, the
MainWindow constructor still proceeds to check the startStopAction_,
which results in MainWindow::startCapture() being called and trying to
use a null camera_ object. Fix this by returning from the constructor as
soon as the error is detected.

This also fixes a similar crash if the camera selection dialog box is
closed without selecting a camera.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Umang Jain <email@uajain.com>

src/qcam/main_window.cpp

@@ -70,8 +70,10 @@ MainWindow::MainWindow(CameraManager *cm, const OptionsParser::Options &options)
 
 	/* Open the camera and start capture. */
 	ret = openCamera();
-	if (ret < 0)
+	if (ret < 0) {
 		quit();
+		return;
+	}
 
 	startStopAction_->setChecked(true);
 }