MDEV-31255: Crash with fulltext search subquery in explain delete

[bsrikanth-mariadb]

ft_handler isn't getting initialized for subqueries inside explain
delete/update queries. However, ft_handler is accessed inside ha_ft_read(),
and is the reason for NULL pointer exception.
This is not the case with non-explain delete/update queries, as
well as explain/non-explain select queries.

Follow the approach the SELECT statements are using in
JOIN::optimize_constant_subqueries(): remove SELECT_DESCRIBE
flag when invoking optimization of constant subqueries.

Single-table UPDATE/DELETEs have SELECT_LEX but don't have JOIN.
So, we make optimize_constant_subqueries() not to be a member
of JOIN class, and instead move it to SELECT_LEX, and then
invoke it from single-table UPDATE/DELETE as well as for SELECT queries.

[spetrunia]

After the patch, the reader will see this in both sql_update and sql_delete:

  if (select_lex->optimize_constant_subqueries())
    DBUG_RETURN(TRUE);

  /* Apply the IN=>EXISTS transformation to all subqueries and optimize them. */
  if (select_lex->optimize_unflattened_subqueries(false))
    DBUG_RETURN(TRUE);

The comment in the middle looks mis-placed.

In both files, make it be

  /* 
    Apply the IN=>EXISTS and other transformations to all subqueries and
    optimize them.

    Constant subqueries are treated in a special way here: they can be
    evaluated even in EXPLAIN statement, so their query plan must be
    fully initialized for computation.
  */
  if (select_lex->optimize_constant_subqueries())
    DBUG_RETURN(TRUE);

  if (select_lex->optimize_unflattened_subqueries(false))
    DBUG_RETURN(TRUE);

[spetrunia]

Ok to push after the above is addressed.