Element_type& Bounds_checked_array<Element_type>::operator[]
(size_t) [with Element_type = Item*; size_t = long unsigned int]

In sql_yacc.yy the semantic actions for the MEDIAN window function
lacked a call of st_select_lex::prepare_add_window_spec().
This function saves the head of the thd->lex->order_list into
lex->save_order_list in order this head to be restored in
st_select_lex::add_window_spec after the specification of the
window function has been parsed.
Without a call of prepare_add_window_spec() when add_window_spec()
was called the head of an empty list was copied into
thd->lex->order_list (instead of assumed saved head this list).
This made the list thd->lex->order_list invalid and potentially
could cause many different problems.

Corrected the result set in the test case for MDEV-15899 that
used the MEDIAN window function and could not be correct
without this fix.