[aron9forever]

Well I guess I can answer my own question here

int is_ent_valid(int iEnt)
{
    if (iEnt < 1 || iEnt > gpGlobals->maxEntities) // <-- shot in the balls here
        return 0;

    if (iEnt <= gpGlobals->maxClients)
    {
        if (!MF_IsPlayerIngame(iEnt))
        {
            return 0;
        }
    } else {
        if (FNullEnt(TypeConversion.id_to_edict(iEnt)))
        {
            return 0;
        }
    }

    return 1;
}

It still seems weird that 0 is not a valid ent, at least in the scope of this implementation / issue. I'll let you decide on that.

pev(1, pev_classname, classname, charsmax(classname)); //works fine if player is valid
pev(0, pev_classname, classname, charsmax(classname)); //works fine
pev(-1, pev_classname, classname, charsmax(classname)); //run time error

pev_valid(1) //returns 2
pev_valid(0) //returns 0
pev_valid(-1) //returns 0 

seems inconsistent to me