- When JOIN::cleanup(full==TRUE) is called, the select can be in two states:
  = Right after the create_sort_index() call, when join->join_tab[0] is used to 
    read data produced by filesort().
  = After create_sort_index(), and after JOIN::reinit() calls, when 
    join->join_tab[0] has been reset to read the original data. 
- We didn't handle the second case correctly, which resulted in an attempt to free 
  the same SQL_SELECT two times. The fix is to make sure we don't double-free.