`changeAttribute`: don't reduce hitpoints of immortal npcs

[thokkat]

The orc in the new mine who makes the Ulu-Mulu would have his health reduced instead of increased when taking his medicine. This is due to a script bug giving him the wrong guild and making this check fail.

    if (C_NpcIsOrc(self))
    {
        self.attribute[ATR_HITPOINTS] = self.attribute[ATR_HITPOINTS_MAX];
    }
    else
    {
        Npc_ChangeAttribute (self,ATR_HITPOINTS,-self.attribute[ATR_HITPOINTS_MAX]);
    };

In vanilla death is prevented by immortality. OG checks immortality only when damage is dealt. This check is now moved to Npc::changeAttribute like it was done for godmode.

[Try]

LGTM, thanks @thokkat !