innodb_monitor_validate(): Let item_val_str() allocate the memory
in THD, so that it will be available to innodb_monitor_update().
In this way, there is no need to allocate another buffer, and
no problem if the call to innodb_monitor_update() is skipped due
to an invalid value that is passed to another configuration parameter.

There are some other callers to st_mysql_sys_var::val_str()
that validate configuration parameters that are related to FULLTEXT INDEX,
but they will allocate memory by invoking thd_strmake().