Fix based on suggestion by Zardosht Kasheff and Richard Prohaska) for making all clustered indexes equal in test_if_skip_sort_order()


sql/sql_select.cc:
  - Threat all clustered indexes equal in test_if_skip_sort_order().
    This is a temporary fix as the current code doesn't do proper cost analyizes for which index to use.
    I will address this later as the change required is not trivial.
  - Fixed a bug where select_limit was changed in the loop, which made different indexes see diferent values of select_limit