Avoid crash when iface remove collides with ACS-COMPLETE

When ACS-COMPLETE is immediately followed by interface removal the actual interface removal happens inside wpa_msg call in hostapd_acs_completed and struct hostapd_iface is freed. That results in further crashes. I have added a check that iface is still present in the interfaces list and its state is still ACS, and return with a new error code if that happens.