Fix a NULL-ptr-deref bug in ath10k_usb_alloc_urb_from_pipe

hulk inclusion
category: bugfix
bugzilla: 13690
CVE: CVE-2019-15099

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The `ar_usb` field of `ath10k_usb_pipe_usb_pipe` objects
are initialized to point to the containing `ath10k_usb` object
according to endpoint descriptors read from the device side, as shown
below in `ath10k_usb_setup_pipe_resources`:

for (i = 0; i < iface_desc->desc.bNumEndpoints; ++i) {
        endpoint = &iface_desc->endpoint[i].desc;

        // get the address from endpoint descriptor
        pipe_num = ath10k_usb_get_logical_pipe_num(ar_usb,
                                                endpoint->bEndpointAddress,
                                                &urbcount);
        ......
        // select the pipe object
        pipe = &ar_usb->pipes[pipe_num];

        // initialize the ar_usb field
        pipe->ar_usb = ar_usb;
}

The driver assumes that the addresses reported in endpoint
descriptors from device side  to be complete. If a device is
malicious and does not report complete addresses, it may trigger
NULL-ptr-deref `ath10k_usb_alloc_urb_from_pipe` and
`ath10k_usb_free_urb_to_pipe`.

This patch fixes the bug by preventing potential NULL-ptr-deref.

Signed-off-by: Hui Peng <benquike@gmail.com>
Reported-by: Hui Peng <benquike@gmail.com>
Reported-by: Mathias Payer <mathias.payer@nebelwelt.net>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>