net: ipconfig: Fix NULL pointer dereference on RARP/BOOTP/DHCP timeout

If no RARP, BOOTP, or DHCP response is received, ic_dev is never set,
causing a NULL pointer dereference in ic_close_devs():

    Sending DHCP requests ...... timed out!
    Unable to handle kernel NULL pointer dereference at virtual address 00000004

To fix this, add a check to avoid dereferencing ic_dev if it is still
NULL.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Fixes: 2647cffb ("net: ipconfig: Support using "delayed" DHCP replies")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ipconfig.c

@@ -306,7 +306,7 @@ static void __init ic_close_devs(void)
 	while ((d = next)) {
 		next = d->next;
 		dev = d->dev;
-		if (dev != ic_dev->dev && !netdev_uses_dsa(dev)) {
+		if ((!ic_dev || dev != ic_dev->dev) && !netdev_uses_dsa(dev)) {
 			pr_debug("IP-Config: Downing %s\n", dev->name);
 			dev_change_flags(dev, d->flags);
 		}