When a table is renamed to an internal #sql2 or #sql-ib name during
a table-rebuilding DDL operation such as OPTIMIZE TABLE or ALTER TABLE,
and shortly after that a purge operation in an index on virtual columns
is attempted, the operation could fail, but purge would fail to release
the table reference.

innodb_acquire_mdl(): Release the reference if the table name is not
valid for acquiring a meta-data lock (MDL).

innodb_find_table_for_vc(): Add a debug assertion if the table name
is not valid. This code path is for DML execution. The table
should have a valid name for executing DML, and furthermore a MDL
will prevent the table from being renamed.

row_vers_build_clust_v_col(): Add a debug assertion that both indexes
must belong to the same table.