A to-be-closed variable must have a closable value (or be nil)

It is an error for a to-be-closed variable to have a non-closable
non-nil value when it is being closed. This situation does not seem to
be useful and often hints to an error. (Particularly in the C API, it is
easy to change a to-be-closed index by mistake.)

lapi.c

@@ -1299,7 +1299,7 @@ static const char *aux_upvalue (TValue *fi, int n, TValue **val,
       *val = f->upvals[n-1]->v;
       if (owner) *owner = obj2gco(f->upvals[n - 1]);
       name = p->upvalues[n-1].name;
-      return (name == NULL) ? "(*no name)" : getstr(name);
+      return (name == NULL) ? "(no name)" : getstr(name);
     }
     default: return NULL;  /* not a closure */
   }