Fix a warning in libusb.

A new clang warns about this:

../../third_party/libusb/src/libusb/os/darwin_usb.c:1271:27: error: expression which evaluates to zero treated as a null pointer constant of type 'IOUSBInterfaceInterface550 **' (aka 'struct IOUSBInterfaceStruct550 **') [-Werror,-Wnon-literal-null-conversion]
  cInterface->interface = IO_OBJECT_NULL;
                          ^~~~~~~~~~~~~~

The warning looks correct to me. I filed upstream ticket 147 for this:
http://libusb.org/moderator/confirm/147 (currently awaiting moderation).

BUG=323668
TBR=cpu

Review URL: https://codereview.chromium.org/111853011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242548 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242548