cdc-acm: fix NULL pointer reference

The union descriptor must be checked. Its usage was conditional
before the parser was introduced. This is important, because
many RNDIS device, which also use the common parser, have
bogus extra descriptors.
Signed-off-by: NOliver Neukum <oneukum@suse.com>
Tested-by: NVasily Galkin <galkin-vv@yandex.ru>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/cdc_ether.c

@@ -160,6 +160,12 @@ int usbnet_generic_cdc_bind(struct usbnet *dev, struct usb_interface *intf)
 	info->u = header.usb_cdc_union_desc;
 	info->header = header.usb_cdc_header_desc;
 	info->ether = header.usb_cdc_ether_desc;
+	if (!info->u) {
+		if (rndis)
+			goto skip;
+		else /* in that case a quirk is mandatory */
+			goto bad_desc;
+	}
 	/* we need a master/control interface (what we're
 	 * probed with) and a slave/data interface; union
 	 * descriptors sort this all out.
@@ -256,7 +262,7 @@ int usbnet_generic_cdc_bind(struct usbnet *dev, struct usb_interface *intf)
 			goto bad_desc;
 		}
 
-	} else if (!info->header || !info->u || (!rndis && !info->ether)) {
+	} else if (!info->header || (!rndis && !info->ether)) {
 		dev_dbg(&intf->dev, "missing cdc %s%s%sdescriptor\n",
 			info->header ? "" : "header ",
 			info->u ? "" : "union ",