fix defaults for preference combos

depending on memory region, last item was always made default. If afterwards removed due to missing capability, this would cause invalid data after reset to default. fixes darktable-org#17065

src/bauhaus/bauhaus.c

@@ -1562,7 +1562,7 @@ void dt_bauhaus_combobox_add_full(GtkWidget *widget,
   g_ptr_array_add(d->entries, entry);
   if(d->active < 0)
     d->active = 0;
-  if(d->defpos < 0 && sensitive)
+  if(d->defpos == -1 && sensitive)
     d->defpos = GPOINTER_TO_INT(data);
 }