Fix a bug in replayWrite() - for some reason, it is occasionally getting called with a length of 0. In this case, write() returning a 0 does not indicate an error.
Fix a bug in replayWrite() - for some reason, it is occasionally
getting called with a length of 0. In this case, write() returning a
0 does not indicate an error.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome