MDEV-25684 Crash in THD::find_temporary_table while calling spider_direct_sql UDF without temporary table created

The server crashed when SPIDER_DIRECT_SQL UDF was called with
non-existing temporary table.

The bug has been introduced by 91ffdc83. The commit removed
the check, from THD::open_temporary_table(), which ensure that
the target temporary tables exist.

We can fix the bug by adding the check before the call of
THD::open_temporary_table().