- Remove 'dummy_for_valgrind' overrun marker as this doesn't help much.
  The element also distorts the sizes of objects a bit, which makes it
  harder to calculate gain in object sizes when doing size optimizations.
- Replace usage of thd_get_current_thd() with _current_thd()
- Avoid one extra call indirection when using thd_get_current_thd(), which
  is used by Sql_alloc, by replacing it with _current_thd()