[Lazy]

I think what your looking for can be found in the quake 1 source...

 int NUM_FOR_EDICT(edict_t *e)
 {
 	int		b;
 	
 	b = (byte *)e - (byte *)sv.edicts;
 	b = b / pr_edict_size;
 	
 	if (b < 0 || b >= sv.num_edicts)
 		Sys_Error ("NUM_FOR_EDICT: bad pointer");
 	return b;
 }

Basically make sure any calls to ENTINDEX are within range.