MDEV-32965:  Assertion `thd->active_stmt_arena_to_use()-> is_stmt_prepare_or_first_sp_execute() || thd->active_stmt_arena_to_use()-> is_conventional() || thd->active_stmt_arena_to_use()->state == Query_arena::STMT_SP_QUERY_ARGUMENTS' failed

This patch fixes too strong condition in assert at the method
  Item_func_group_concat::fix_fields
that is true in case of a stored routine and obviously broken
for a prepared statement.