Review fix for Bug#39696, serial log corruption

[Christopher Powers]

Kevin,

Can you have a look at this commit: http://lists.mysql.com/commits/58261 ?

The falcon_chill_thaw stress test now completes on Xeno when it has 
never done so before. I've pushed the fix to falcon-chris to see how it 
does on Pushbuild2.

SerialLog::flush() bears a bit more scrutiny, and should be reviewed to 
see how it handles zero-byte flushes.

Chris