Fix a warning in libusb.

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=242548

[Daniel Erat, 2013-12-26 16:41:32]

lgtm

[commit-bot: I haz the power, 2013-12-26 16:42:04]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/thakis@chromium.org/111853011/40001

[commit-bot: I haz the power, 2013-12-26 17:02:52]

Retried try job too often on chromium_presubmit for step(s) presubmit
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=chromium_p...

[commit-bot: I haz the power, 2013-12-26 17:05:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/thakis@chromium.org/111853011/40001

[commit-bot: I haz the power, 2013-12-26 19:49:45]

Change committed as 242548