libusb: consider bad fd as a broken event

Application can use libusb_get_pollfds to get pollfds from libusb then
close the fd themselves. This cause the hotplug thread unable to leave
because it will be consider as a invalid event then loop forever instead
of a broken event that should be quit immediately.

Reviewed by: bapt
Sponsored by: FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50959