darwin: fix SEGV on libusb_close after failed re-enumeration

This commit fixes an issue that can occur if the user calls libusb_close after a failed attempt
of libusb_reset (re-enumerate). This can lead to an attempt to dereference dpriv->device which
was set to NULL during the re-enumeration.

Signed-off-by: Nathan Hjelm <hjelmn@google.com>