linux_udev: Fix a Coverity warning

linux_udev: Fix a Coverity warning

This really is a false positive, but easy enough to silence:

*** CID 62578:  Failure to restore non-local value  (MISSING_RESTORE)
/libusb/os/linux_udev.c: 64 in linux_udev_start_event_monitor()
58     	int r;
59
60     	assert(udev_ctx == NULL);
61     	udev_ctx = udev_new();
62     	if (!udev_ctx) {
63     		usbi_err(NULL, "could not create udev context");
>>>     CID 62578:  Failure to restore non-local value  (MISSING_RESTORE)
>>>     Value of non-local "udev_ctx" that was verified to be "NULL" is not restored as it was along other paths.
64     		return LIBUSB_ERROR_OTHER;
65     	}
66
67     	udev_monitor = udev_monitor_new_from_netlink(udev_ctx, "udev");
68     	if (!udev_monitor) {
69     		usbi_err(NULL, "could not initialize udev monitor");

Signed-off-by: Hans de Goede <hdegoede@redhat.com>