There are two calls to read_log_event() on master in mysql_binlog_send().
Each call reads 19 bytes in this test case and the error of the second
read_log_event() is reported to the slave.

The second read_log_event() starts from position 94 (75 + 19) to 113
(75 + 19 + 19). Usually, there are two events in the binary log:

    . 0   - 3   - Header
    . 4   - 105 - Format Descriptor Event
    . 106 - 304 - Query Event

and both reads fail because operations are reading from invalid positions
as expected.

However, mysql_binlog_send() does not use the same IO_CACHE that is used to
write into binary log (i.e. mysql_bin_log.log_file) for the hot binary log.
It opens the binary log file directly by calling open_binlog() and creates a
separated IO_CACHE. So there is a possibly that after a master has flushed
the binary log file, the content has been cached by the filesystem, and has
not updated the disk file. If this happens, then a slave will only see part
of the file, and thus the second read_log_event() will report event truncated
error.

To fix the problem, if the first read_log_event() has failed, we ensure that
the second one will try to read from the same position.