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>

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My apologies for the breakage. I tested only one target.