NULL pointer access in acd_disable() if order of calls with acd_enable() is not well-formed

If LwIP_ACD_Create() is called, followed by LwIP_ACD_Disable(), the IP stack writes to a NULL pointer in acd_disable():

netif->acd->defendActive = 0;

Since netif->acd was never set to a valid address.
Add a NULL pointer test before accessing the member.