Allow find_ent_by_owner native to work in client_disconnect forward

Reported by beneado at https://forums.alliedmods.net/showthread.php?t=246902.

This allows ```find_ent_by_owner``` native to work in ```client_disconnect``` forward as at this point player is not yet fully disconnected and player's edict is still valid. 

Difference between ```CHECK_ENTITY``` and ```CHECK_ENTITY_SIMPLE``` is the first checks entity is in-game (from AMXX point of view). AMXX keeps the current player's edict until a new player is connecting.

This should be safe change.

@Nextra