oops; missed a file on the last commit.  Minor change to disallow empty mbox names.
oops; missed a file on the last commit.  Minor change to disallow empty mbox names.

nmathewson/mixminion,reezer/mixminion,Javex/mixminion,mixminion/mixminion,rxcomm/mixminion,reezer/mixminion,crooks/mixminion,Javex/mixminion,crooks/mixminion,reezer/mixminion,reezer/mixminion,mixminion/mixminion,rxcomm/mixminion,crooks/mixminion,crooks/mixminion,mixminion/mixminion,nmathewson/mixminion,nmathewson/mixminion,rxcomm/mixminion,Javex/mixminion,Javex/mixminion,nmathewson/mixminion,mixminion/mixminion,rxcomm/mixminion