Assertion "log_sys.n_pending_flushes == 1" fails in undo_truncate test, on shutdown

http://buildbot.askmonty.org/buildbot/builders/winx64-packages/builds/27562/steps/test/logs/stdio

Looks like one thread is doing log_write_and_flush(), while another is doing log_write_flush_to_disk_low() . Also and third one is doing close_connections(), so perhaps this could be related to the recent sudden surge of crashes on shutdown in this specific test.