- make merge_buffers():sort_length have type size_t as this type is 
  expected by, e.g. ptr_compare_1, which will receive pointer to 
  sort_length as comparison parameter.


sql/filesort.cc:
  BUG#29610: crash in func_group on 64bit platform:
  - make sort_length have type size_t as this type is expected by, e.g.
    ptr_compare_1, which will receive pointer to sort_length as comparison
    parameter.