MDEV-31296: Crash in Item_func::fix_fields when prepared statement with subqueries and window function is executed with sql_mode = ONLY_FULL_GROUP_BY

Crash was caused by referencing a null pointer on getting
the number of the nesting levels of the set function for the current
select_lex at the method Item_field::fix_fields.

The current select for processing is taken from Name_resolution_context
that filled in at the function set_new_item_local_context() and
where initialization of the data member Name_resolution_context
was mistakenly removed by the commit
  d6ee351b
   (Revert "MDEV-24454 Crash at change_item_tree")

To fix the issue, correct initialization of data member
  Name_resolution_context::select_lex
that was removed by the commit d6ee351b
is restored.