Bogus 'Event truncated', 'Event too big', 'Event invalid' upon SHOW BINLOG EVENTS FROM <pos>

All attached test cases end with SHOW BINLOG EVENTS FROM 10000, which in all cases causes ER_ERROR_WHEN_EXECUTING_COMMAND with the text Wrong offset or I/O error. Maybe the offset is really wrong, but I don't suppose it should produce the errors in the error log hinting at corruption:

2019-01-06 20:48:36 5 [ERROR] Error in Log_event::read_log_event(): 'Event truncated'

2019-01-06 20:49:29 5 [ERROR] Error in Log_event::read_log_event(): 'Event too big', data_len: 1667591269, event_type: 101

2019-01-06 20:49:55 5 [ERROR] Error in Log_event::read_log_event(): 'Event invalid', data_len: 0, event_type: 52

Certainly there are no big events in the binary logs, and I don't think any of them are invalid.

I've set the affects versions to 10.3+, because the test case is essentially 10.3-specific. It's very likely that the failures happen in earlier versions as well. I don't think, however, that it's critical to fix them in prior GA releases.