fix issue #1469 - glitches with type-naming

Issue reported by ars3niy:  assigning names to types of objects,
or clearing such, did not update persistent inventory window.  Also,
the sequence
  assign-a-name,
  name-as-' '-to-unname,
  assign-a-name again,
  unname again
would result in impossible: "named object not in disco".

This fixes the impossibility.  The fix for #1470 has already taken
care of the presistent inventory issue.

Fixes #1469

src/o_init.c

@@ -488,7 +488,7 @@ discover_object(
 void
 undiscover_object(int oindx)
 {
-    if (!objects[oindx].oc_name_known) {
+    if (!objects[oindx].oc_name_known && !objects[oindx].oc_encountered) {
         int dindx, acls = objects[oindx].oc_class;
         boolean found = FALSE;
 
@@ -510,7 +510,6 @@ undiscover_object(int oindx)
 
         if (objects[oindx].oc_class == GEM_CLASS)
             gem_learned(oindx); /* ok, it's actually been unlearned */
-        update_inventory();
     }
 }