The problem here is when we have a materialized derived table that
is determined to be superfluous, thus destroy_materialized() is
called on it. Later, the function find_table_in_global_list() is
called to search for duplicate table names for the table to be
updated. When it finds the derived table, it is in an inconsistent
state (table->table != NULL but table->table->s undefined).

The solution is to let be the derived table's TABLE_LIST object
remain in a consistent state after its materialized object is deleted,
by setting its TABLE pointer to NULL.