libusb_init causes segfault

[Akos V. <ax...@gm...>]

Hi!

If libusb_init is supplied with a nil pointer, it causes a segfault. Even
though this is a user application error, it should be checked, and an error
should be returned.

The fix is quite trivial I guess.

The code which causes the segafult:

libusb_context *x = NULL;
libusb_init(&x);

Regards,
  Ákos Vandra