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: Geert Uytterhoeven <geert+renesas@glider.be>
Fixes: 2647cffb ("net: ipconfig: Support using "delayed" DHCP replies")
Signed-off-by: David S. Miller <davem@davemloft.net>