Fix bug #8677: if one used LOCK TABLES, created an InnoDB temp table, and did a multi-table update where a MyISAM table was the update table and the temp table was a read table, then InnoDB aserted in row0sel.c because n_mysql_tables_in_use was 0. Also, we remove the assertion altogether and just print an error to the .err log if this important consistency check fails. Then it is up to the user to read the .err log and notice the problem if there still are errors in MySQL's table locking.