[dhcp] Ignore DHCPACKs containing incorrect IP addresses

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/dhcp.c

 		return;
 	if ( server_id.s_addr != dhcp->server.s_addr )
 		return;
+	if ( ip.s_addr != dhcp->offer.s_addr )
+		return;
 
 	/* Record assigned address */
 	dhcp->local.sin_addr = ip;