MDEV-20589: Server still crashes in Field::set_warning_truncated_wrong_value

- Use dbug_tmp_use_all_columns() to mark that all fields can be used
- Remove field->is_stat_field (not needed)
- Remove extra arguments to Field::clone() that should not be there
- Safety fix for Field::set_warning_truncated_wrong_value() to not crash
  if table is zero in production builds (We have got crashes several times
  here so better to be safe than sorry).
- Threat wrong character string warnings identical to other field
  conversion warnings. This removes some warnings we before got from
  internal conversion errors.  There is no good reason why a user would
  get an error in case of 'key_field='wrong-utf8-string' but not for
  'field=wrong-utf8-string'.  The old code could also easily give
  thousands of no-sence warnings for one single statement.