objects_port_set_activating_cluster takes the cluster a co-op host sends
in its presentation (network_coop.c), and refused only NONE and indices
past the BSP's clusters. Any other negative value (activating_cluster is
sent plus one, so a host sending -5 or 0x8001) was stored, and the client
then read the cluster PVS before the start of its array. Any negative
index now means none, as NONE already did.