Check whether libusb_init() succeeds, and give up if it doesn't.

Check whether libusb_init() succeeds, and give up if it doesn't.

libusb_init() isn't guaranteed to succeed; it returns a (presumably)
non-zero error on failure.  If it doesn't return 0, just give up - we
presumably don't have a valid libusb_context pointer, which is probably
what's causing crashes when we call libusb_get_device_list() on a system
with no usbfs mounted.

This fixes bug 3554749 in the tracker, and a bunch of upstream bugs.