m_status == DA_OK

Reading from information_scema.tables or information_schema.columns
may cause assertion failure in debug builds. This may happen under
rare circumstances when information_schema fails to get information
about a table (e.g. when a connection is killed).

This happens because open_normal_and_derived_tables() can return an
error without setting an error message in THD. But information_schema
attempts to get an error message from THD unconditionally.

With this fix information_schema attempts to get an error message
from THD only in case error message is set in THD.