MDEV-13708 Crash with indexed virtual columns and FK cascading deletes

InnoDB was too eager to forget the open table (m_mysql_table=NULL)
and that caused it to try to open a table which was opened by the user
not FK-prelocked. The server didn't expect that.

After fixing this, it crashed in gcol.innodb_virtual_fk test, trying to
compute virtual columns for a table that didn't have them. Because
row_upd_store_row() was deleting a row from node->table, while computing
virtual columns in thr->prebuilt->m_mysql_table. Which wasn't necessarily
the same table, and might've not even had virtual columns, even if
node->table did.

mysql-test/suite/vcol/r/innodb_virtual_fk.result

+set default_storage_engine=innodb;
+create table t1 (id int primary key, id2 int as (id) virtual, key id2 (id2));
+create table t2 (id int key, constraint fk_id foreign key (id) references t1 (id) on delete cascade);
+insert into t1 (id) values (1), (2);
+insert into t2 (id) values (1), (2);
+delete from t1;
+select * from t1;
+id	id2
+select * from t2;
+id
+drop table t2;
+drop table t1;

mysql-test/suite/vcol/t/innodb_virtual_fk.test

+source include/have_innodb.inc;
+set default_storage_engine=innodb;
+
+#
+# MDEV-13708 Crash with indexed virtual columns and FK cascading deletes
+#
+
+create table t1 (id int primary key, id2 int as (id) virtual, key id2 (id2));
+create table t2 (id int key, constraint fk_id foreign key (id) references t1 (id) on delete cascade);
+insert into t1 (id) values (1), (2);
+insert into t2 (id) values (1), (2);
+delete from t1;
+select * from t1;
+select * from t2;
+drop table t2;
+drop table t1;

storage/innobase/row/row0mysql.cc

@@ -2029,7 +2029,6 @@ row_update_for_mysql(row_prebuilt_t* prebuilt)
 		node->cascade_upd_nodes = cascade_upd_nodes;
 		cascade_upd_nodes->pop_front();
 		thr->fk_cascade_depth++;
-		prebuilt->m_mysql_table = NULL;
 
 		goto run_again;
 	}

storage/innobase/row/row0upd.cc

@@ -2966,7 +2966,8 @@ row_upd_del_mark_clust_rec(
 	entries */
 
 	row_upd_store_row(node, trx->mysql_thd,
-			  thr->prebuilt ? thr->prebuilt->m_mysql_table : NULL);
+			  thr->prebuilt  && thr->prebuilt->table == node->table
+			  ? thr->prebuilt->m_mysql_table : NULL);
 
 	/* Mark the clustered index record deleted; we do not have to check
 	locks, because we assume that we have an x-lock on the record */