[IPV4]: ipconfig.c: fix dhcp timeout behaviour

I think there is a small bug in ipconfig.c in case IPCONFIG_DHCP is set
and dhcp is used.

When a DHCPOFFER is received, ip address is kept until we get DHCPACK.
If no ack is received, ic_dynamic() returns negatively, but leaves the
offered ip address in ic_myaddr.

This makes the main loop in ip_auto_config() break and uses the maybe
incomplete configuration.

Not sure if it's the best way to do, but the following trivial patch
correct this. 
Signed-off-by: NMaxime Bizon <mbizon@freebox.fr>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ipconfig.c

@@ -1149,8 +1149,10 @@ static int __init ic_dynamic(void)
 		ic_rarp_cleanup();
 #endif
 
-	if (!ic_got_reply)
+	if (!ic_got_reply) {
+		ic_myaddr = INADDR_NONE;
 		return -1;
+	}
 
 	printk("IP-Config: Got %s answer from %u.%u.%u.%u, ",
 		((ic_got_reply & IC_RARP) ? "RARP"