buf_flush_page_cleaner_coordinator: In the first loop, use an
appropriate termination condition, waiting for !recv_writer_thread_active.

logs_empty_and_mark_files_at_shutdown(): Signal recv_sys->flush_start
in case the recv_writer_thread was never started, or
buf_flush_page_cleaner_coordinator failed to notice its termination.

innobase_start_or_create_for_mysql(): Remove a redundant, unreachable
condition, and properly release resources when aborting startup due to
recv_sys->found_corrupt_log.