If is not so easy to get out of zlib how many bytes have been written actually. I used an undocumented function, 
which breaks down on 64-bit systems.

I now rewrote the code in mlogger.cxx to use lseek() to "measure" actually the output file and set the values 
correctly. I tried on a few systems but am not 100% sure if it works everywhere. Can you please double check?

The fix is in SVN revision 5347.

/Stefan