Fixed compiler warnings.

include/queues.h:
  Added prototype for queue_replace()
mysys/queues.c:
  Fixed wrong queue_replace()
mysys/thr_alarm.c:
  Added DBUG_PRINT
sql/item_subselect.cc:
  Check return value of ha_rnd_init().
  (Fixes compiler warnings)
sql/sql_class.cc:
  Fixed wrong test
sql/sql_show.cc:
  Removed not used variable.