Underlying causes of all bugs mentioned below are same. This patch fixes
all of them:
1) MDEV-25028: ASAN use-after-poison in
base_list_iterator::next or Assertion `sl->join == 0' upon
INSERT .. RETURNING via PS
2) MDEV-25187: Assertion `inited == NONE || table->open_by_handler'
failed or Direct leak in init_dynamic_array2 upon INSERT .. RETURNING
and memory leak in init_dynamic_array2
3) MDEV-28740: crash in INSERT RETURNING subquery in prepared statements
4) MDEV-27165: crash in base_list_iterator::next
5) MDEV-29686: Assertion `slave == 0' failed in
st_select_lex_node::attach_single

Analysis:
consider this statement:
INSERT(1)...SELECT(2)...(SELECT(3)...) RETURNING (SELECT(4)...)

When RETURNING is encountered, add_slave() changes how selects are linked.
It makes the builtin_select(1) slave of SELECT(2). This causes
losing of already existing slave(3) (which is nested select of SELECT of
INSERT...SELECT). When really, builtin_select (1) shouldn't be slave to
SELECT(2) because it is not nested within it. Also, push_select() to use
correct context also changed how select are linked.
During reinit_stmt_before_use(), we expect the selects to
be cleaned-up and have join=0. Since these selects are not linked correctly,
clean-up doesn't happen correctly so join is not NULL. Hence the crash.

Fix:
IF we are parsing RETURNING, make is_parsing_returning= true for
current select. get rid of add_slave(). In place of push_select(), used
push_context() to have correct context (the context of builtin_select)
to resolve items in item_list. And add these items to item_list of
builtin_select.