zd1211rw, fix oops when ejecting install media

patch e0579d57 in mainline.

The disconnect function can dereference the net_device structure when it
is never allocated. This is the case when ejecting the device installer.
Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
Acked-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/net/wireless/zd1211rw/zd_usb.c

@@ -1041,14 +1041,17 @@ error:
 static void disconnect(struct usb_interface *intf)
 {
 	struct net_device *netdev = zd_intf_to_netdev(intf);
-	struct zd_mac *mac = zd_netdev_mac(netdev);
-	struct zd_usb *usb = &mac->chip.usb;
+	struct zd_mac *mac;
+	struct zd_usb *usb;
 
 	/* Either something really bad happened, or we're just dealing with
 	 * a DEVICE_INSTALLER. */
 	if (netdev == NULL)
 		return;
 
+	mac = zd_netdev_mac(netdev);
+	usb = &mac->chip.usb;
+
 	dev_dbg_f(zd_usb_dev(usb), "\n");
 
 	zd_netdev_disconnect(netdev);