clean up spider code patterns that could result in -wmaybe-uninitialized

  if (

    !(db_name = get_field(mem_root, table->field[0])) ||

    !(table_name = get_field(mem_root, table->field[1])) ||

    !(link_id = get_field(mem_root, table->field[2]))

  )

    DBUG_RETURN(HA_ERR_OUT_OF_MEM);

if (!(db_name = get_field(...))

  DBUG_RETURN(...);

if (!(table_name = get_field(...))

  DBUG_RETURN(...);

if (!(link_id = get_field(...))

  DBUG_RETURN(...);