BUG#29610: crash in func_group on 64bit platform:

- 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.

sql/filesort.cc

@@ -1120,7 +1120,8 @@ int merge_buffers(SORTPARAM *param, IO_CACHE *from_file,
                   int flag)
 {
   int error;
-  uint rec_length,sort_length,res_length,offset;
+  uint rec_length,res_length,offset;
+  size_t sort_length;
   ulong maxcount;
   ha_rows max_rows,org_max_rows;
   my_off_t to_start_filepos;