When a hotplug callback has been registered, and the program using
libusb is calling libusb_exit then the thread handler is set to
NO_THREAD which result in the variable controlling the loop the be set
to 0, it does a last pass through device available without having done
a scan, which result in a sigbus after it tried to unregister all the
devices.
directly break the loop instead and cleanup the list of devices
this fixes the tests with LGPLed libusb's hotplugtest program