Fix cg.skulltrails out of bounds access in Team Arena Harvester mode

In Team Arena's Harvester mode, players corrupt your memory from beyond
the grave. Gib the players to stop the corruption!

CG_PlayerTokens is called for player entities, including corpses.
The entity number is used for the index in cg.skulltrails which only has
MAX_CLIENTS elements. This results in incorrect memory being overwritten
for corpse entities (as the entity number is >= MAX_CLIENTS).

So limit skull trails to valid entities (entity number < MAX_CLIENTS).