MDEV-23294 Segfault or assertion upon MyISAM repair

When computing vcol expression some items use current_thd and that was
not set in MyISAM repair thread. Since all the repair threads belong
to one connection and items should not write into THD we can utilize
table THD for that.

include/myisamchk.h

@@ -117,6 +117,7 @@ typedef struct st_handler_check_param
   uint progress_counter;             /* How often to call _report_progress() */
   ulonglong progress, max_progress;
 
+  void (*init_fix_record)(void *);
   int (*fix_record)(struct st_myisam_info *info, uchar *record, int keynum);
 
   mysql_mutex_t print_msg_mutex;