It is possible that an object that was originally created by
open_purge_table() will remain cached and reused for SQL execution.
Our previous fix wrongly assumed that ha_innobase::open() would
always be called before SQL execution starts. Therefore, we must
invoke dict_stats_init() in ha_innobase::info_low() instead of
only doing it in ha_innobase::open().

Note: Concurrent execution of dict_stats_init() on the same table
is possible, but it also was possible between two calls to
ha_innobase::open(), with no ill effects observed.

This should fix the assertion failure on stat_initialized.
A possibly easy way to reproduce it would have been
to run the server with innodb_force_recovery=2 (disable the purge of
history), update a table so that an indexed virtual column will be
affected, and finally restart the server normally (purge enabled),
to observe a crash when the table is accessed from SQL.

The problem was first observed and this fix verified by
Elena Stepanova. Also Thirunarayanan Balathandayuthapani
repeated the problem.