fix CHAP authentication

Empty discovery and login packets are legal, and have the same behavior
as packets with a single NUL in them.  Introduced by commit 94d73fc
(Merge pull request #83 from bonzini/coverity, 2013-11-05).

Reported-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Merge pull request #84 from bonzini/master

fix CHAP authentication