Don't discover generic objects

Potion of object detection and then looking at a generic object glyph
on the map would try to "discover" the generic object, and the loop
in discover_object would go too far, overwriting the exclusion_zones
pointer.

Take the easy route and just prevent discovery of generic objects;
there are probably other places where the generic objects should
be handled too, but the fuzzer hasn't hit them.

src/o_init.c

@@ -451,6 +451,9 @@ discover_object(
     boolean mark_as_encountered,
     boolean credit_hero)
 {
+    if (oindx < FIRST_OBJECT) /* don't discover generic objects */
+        return;
+
     if ((!objects[oindx].oc_name_known && mark_as_known)
         || (!objects[oindx].oc_encountered && mark_as_encountered)
         || (Role_if(PM_SAMURAI)