libcamera: udev: Catch udev notification errors

The udev_monitor_receive_device() can return NULL on an error as
detailed in the man pages for the function.

The udevNotify() handler in the DeviceEnumeratorUdev directly uses the
return value of udev_monitor_receive_device() in successive calls to
process the event without having first checked the udev_device.

Ensure we identify, and handle events where the udev_device can not be
returned successfully.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=230
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/device_enumerator_udev.cpp

@@ -332,6 +332,14 @@ int DeviceEnumeratorUdev::addV4L2Device(dev_t devnum)
 void DeviceEnumeratorUdev::udevNotify()
 {
 	struct udev_device *dev = udev_monitor_receive_device(monitor_);
+	if (!dev) {
+		int err = errno;
+		LOG(DeviceEnumerator, Warning)
+			<< "Ignoring notfication received without a device: "
+			<< strerror(err);
+		return;
+	}
+
 	std::string_view action(udev_device_get_action(dev));
 	std::string_view deviceNode(udev_device_get_devnode(dev));