TSan ratelim report

WARNING: ThreadSanitizer: heap-use-after-free (pid=4275)
  Read of size 8 at 0x7b54000066d0 by main thread:
    #0 event_del_ /home/runner/work/libevent/libevent/event.c:2813:32 (libevent_core-2.2.so.1+0x480bf)
    #1 event_del /home/runner/work/libevent/libevent/event.c:2830:9 (libevent_core-2.2.so.1+0x44a8d)
    #2 delete_all_in_dlist /home/runner/work/libevent/libevent/evmap.c:672:3 (libevent_core-2.2.so.1+0x57b00)
    #3 evmap_io_delete_all_iter_fn /home/runner/work/libevent/libevent/evmap.c:681:9 (libevent_core-2.2.so.1+0x55849)
    #4 evmap_io_foreach_fd /home/runner/work/libevent/libevent/evmap.c:564:12 (libevent_core-2.2.so.1+0x551c4)
    #5 evmap_delete_all_ /home/runner/work/libevent/libevent/evmap.c:696:2 (libevent_core-2.2.so.1+0x5578a)
    #6 event_base_free_ /home/runner/work/libevent/libevent/event.c:885:2 (libevent_core-2.2.so.1+0x39eff)
    #7 event_base_free /home/runner/work/libevent/libevent/event.c:979:2 (libevent_core-2.2.so.1+0x3957d)
    #8 test_ratelimiting /home/runner/work/libevent/libevent/test/test-ratelim.c:552:2 (test-ratelim+0x4b685e)
    #9 main /home/runner/work/libevent/libevent/test/test-ratelim.c:687:9 (test-ratelim+0x4b47bb)

  Previous write of size 8 at 0x7b54000066d0 by main thread:
    #0 free <null> (test-ratelim+0x424698)
    #1 event_mm_free_ /home/runner/work/libevent/libevent/event.c:3608:3 (libevent_core-2.2.so.1+0x366a6)
    #2 bufferevent_finalize_cb_ /home/runner/work/libevent/libevent/bufferevent.c:787:2 (libevent_core-2.2.so.1+0x25549)
    #3 event_process_active_single_queue /home/runner/work/libevent/libevent/event.c:1742:4 (libevent_core-2.2.so.1+0x4f320)
    #4 event_process_active /home/runner/work/libevent/libevent/event.c:1805:9 (libevent_core-2.2.so.1+0x43168)
    #5 event_base_loop /home/runner/work/libevent/libevent/event.c:2047:12 (libevent_core-2.2.so.1+0x40cef)
    #6 event_base_dispatch /home/runner/work/libevent/libevent/event.c:1839:10 (libevent_core-2.2.so.1+0x4001a)
    #7 test_ratelimiting /home/runner/work/libevent/libevent/test/test-ratelim.c:480:3 (test-ratelim+0x4b5d74)
    #8 main /home/runner/work/libevent/libevent/test/test-ratelim.c:687:9 (test-ratelim+0x4b47bb)

https://github.com/libevent/libevent/runs/3643967041?check_suite_focus=true

Start with the TSan report and test/test-ratelim.c, especially the paths at lines 480 and 552. Trace the cleanup flow through event.c, evmap.c, and bufferevent.c to understand the reported use-after-free. Done means test-ratelim no longer produces this TSan failure during event-base cleanup.