MDEV-24935: Server crashes in Field_iterator_natural_join::next or Field_iterator_table_ref::set_field_iterator upon 2nd execution of SP

Calling a stored routine that executes a join on three or more tables
and referencing not-existent column name in the USING clause resulted in
a crash on its second invocation.

Server crash taken place by the reason of dereferencing null pointer
in condition of DBUG_ASSERT inside the method
Field_iterator_natural_join::next()
There the data member
  cur_column_ref->table_field->field
has the nullptr value that was reset at the end of first
execution of a stored routine when the standalone procedure
cleanup_items() called by the method sp_head::execute.
Later this data member is not re-initialized and never referenced
in any place except the DBUG_ASSERT on second and later invocations
of the stored routine.

To fix the issue, the assert's condition should be augmented by
a condition '|| !cur_column_ref->table_field' before dereferencing
cur_column_ref->table_field. Such extra checking is aligned with
conditions used by DBUG_ASSERT macros used by implementation of
the class Field_iterator_table_ref that aggregated the class
Field_iterator_natural_join.

sql/table.cc

@@ -6948,6 +6948,7 @@ void Field_iterator_natural_join::next()
 {
   cur_column_ref= column_ref_it++;
   DBUG_ASSERT(!cur_column_ref || ! cur_column_ref->table_field ||
+              !cur_column_ref->table_field->field ||
               cur_column_ref->table_ref->table ==
               cur_column_ref->table_field->field->table);
 }