Don't unreference usbi_default_context until it's about to be freed
Don't unreference usbi_default_context until it's about to be freed

If the user is running with the default context, then it seems to me that it's
expected that libusb can be debugged/traced up to the point were libusb_exit
has completed, which isn't possible unless usbi_default_context isn't set to
NULL until the call to free(ctx) is about to be called

pbatard/libusbx-hp,pbatard/libusbx-hp,pbatard/libusbx-hp