Trigger must not be effective when being damaged if it does not take damage

This fixes issue with vehicles damaging triggers by touching them (like the ClickItemTrigger on m5l3 with tanks)

code/fgame/trigger.cpp

@@ -766,6 +766,10 @@ void Trigger::DamageEvent(Event *ev)
     Vector  dir;
     Vector  momentum;
 
+    if (!takedamage) {
+        return;
+    }
+
     if (Immune(ev->GetInteger(9))) {
         // trigger is immune
         return;