usb: core: Prevent null pointer dereference in update_port_device_state

In the Linux kernel, the following vulnerability has been resolved:

usb: core: Prevent null pointer dereference in update_port_device_state

Currently, the function update_port_device_state gets the usb_hub from

udev->parent by calling usb_hub_to_struct_hub.

However, in case the actconfig or the maxchild is 0, the usb_hub would

be NULL and upon further accessing to get port_dev would result in null

pointer dereference.

Fix this by introducing an if check after the usb_hub is populated.