Possible source of segfaults with flush

[Jerry DeLisle]

I was reviewing transfer.c for use of flush.  I have found about three other 
places where we could potentially call 'flush' on an internal unit.  Flush 
assumes it is given a stream to work on and calls 'write'.  Write has nothing to 
do with internal units.  I already fixed one of these to resolve segfaults I was 
seeing here.

I am testing a patch now.  I just wanted to let people know in case they are 
seeing bad behavior in places like Tonto.

Paul,  I don't know if this is what you are seeing or not, but I will fix it anyway.

Regards,

Jerry